Writing sideboard guides for Modern is always a daunting task because there's such a wide variety of decks and no matter how many you include, someone is going to ask you about one you didn't. With so much to get through, I'm going to dispense with my typically loquacious introductions and get right into it.
The Current List
No sideboard guide would be complete without a list, so here's what I would submit for a Modern tournament right now:
This is very close to my winning list from the Open in Baltimore, with some cosmetic changes. The third Thought Scour is a recognition that Chart a Course continued to underperform and without Fiery Temper in the deck, you don't need many discard outlets. The fact that Thought Scour often finds a Faithless Looting to flashback even mitigates the loss there and the difference between one and two mana is huge in a deck that's trying to cast so many spells in such a short time frame. As for the loathesome Izzet Charm, as much as I hate it, the first copy continues to do just enough to keep its spot.
The Rending Volley in the sideboard is a nod to the mirror where Thing in the Ice is the most important card. It's still effective against Humans and Spirits, especially the latter since they will often save their Mausoleum Wanderer to hit the last card in a chain, at which point you can surprise them with an uncounterable spell.
And lastly, the singleton Spell Pierce in the sideboard overperformed enough that I'm now favoring it in the split with Dispel. The latter is better against Burn and Storm, but Spell Pierce can come in against various Big Mana decks and Ironworks, which are matchups I'm scared of. It's also the best card to insulate yourself against a Teferi, Hero of Dominaria against control.
- Monastery Swiftspear's value is dependent on it being able to attack unabated on the early turns. As such, it's weak and frequently cut in creature matchups, where it's easily blocked and matchups where Lightning Bolt and other red removal is common.
- The value of zero-mana spells in this deck is very high, so you should consider leaving in Gut Shots or Surgical Extraction even if the effect is marginal, unless you're expecting a longer game where the speed isn't as valuable.
- Against tax effects like Damping Sphere or Thalia, Guardian of Thraben, hands that are nothing but cantrips become a liability, so consider trimming one or two.
- While I don't advise bringing graveyard hate in against this deck, plenty of players continue to do so. Against Leyline of the Void and Rest in Peace, you can consider trimming an Arclight Phoenix and/or a Faithless Looting/Izzet Charm since those cards become liabilities against the enchantment hate. Against a player overloaded on Surgical Extraction/Ravenous Trap, consider Dispel/Spell Pierce if either is otherwise functional in the matchup or simply trim on Arclight Phoenix and let them mulligan.
- While this deck can effectively play a control game on the early turns, it doesn't have the card advantage of Jeskai or Azorius Control, so you need to be able to turn the corner quickly. Don't cut down your creature count too far or you'll find yourself spinning your wheels with cantrips while ahead and giving your opponent time to draw back into the game
The Sideboard Guide
VS Humans/Bant Spirits
Try hard to keep their battlefield clear on the opening turns of the game, as mitigating the effectiveness of Reflector Mage is important here. This is most often done via removal, but it can also happen by putting them on the backfoot with a fast Arclight Phoenix draw. Once you've done this, transition to pressuring their life total as quickly as possible, since neither of these decks wants to play from behind.
Thing in the Ice and Anger of the Gods are your tools to catch up should you be the one who falls behind, but don't be afraid to use them aggressively since waiting on Thing runs it into Reflector Mage, and waiting on Anger lets them work to insulate their battlefield from it. Thing in the Ice's trigger is greatly mitigated by Aether Vial, so put a premium on targeting them with your Abrades if you're on a Thing draw.
VS Jund/Golgari Midrange
Arclight Phoenix typically gives you inevitability in these matchups, though Kalitas, Traitor of Ghet and Scavenging Ooze can change that calculus. You don't want to be blindly aggressive here since they have the removal and other disruption to catch up from behind, just play it like a midrange mirror and try to extract the most value you can from each card.
VS Azorius Control/Jeskai
These matchups are very flexible in how you sideboard due to the presence of Baneslayer Angel/Lyra Dawnbringer. Either can take over a game, but if you're able to Lightning Axe them, it's a huge tempo swing. With Faithless Looting to discard superfluous copies and Celestial Colonnade as a secondary target, I like to be more conservative and leave both in since most lists have two Angels. If you know they only have one, I'd trim a Lightning Axe and leave an extra threat in, and if they have none, you can trim both and leave all the threats in.
I listed both Thing in the Ice and Monastery Swiftspear because I generally cut Thing against Azorius and Swiftspear against Jeskai as the first threat to go because of how Swiftspear matches up against red removal. Outside of that, it's a better lategame topdeck and gives you a more haste threats to pressure planeswalkers.
The control matchup is all about how much traction they're getting in the game. As long as they aren't activating Azcanta, the Sunken Ruin or a planeswalker to generate card advantage, you can play it like you would Jund or Golgari Midrange, gaining value whenever possible and trying to make their disruption awkward. If they're gaining card advantage, you must be more aggressive and try to end the game.
Despite this dichotomy, I'm aggressive with my counterspells, because early pressure on their life total makes it harder to find a safe spot to land a planeswalker or Search for Azcanta, and Spell Pierce loses value as the game goes on. Tagging an early Path to Exile also makes a juicy target for Surgical Extraction, since without it they will find it difficult to answer your threats.
VS I ronworks
I've gone back and forth on whether or not to bring in the Rending Volley to answer Sai, Master Thopterist and I'm still unsure. Thing in the Ice is a good answer to a battlefield of creature tokens, but you're not in a position to wait on transforming it. It may be better than the last Lightning Bolt or an Abrade, but it's close.
Use your Spell Pierces and Abrades aggressively, especially on their mana acceleration, because you'll generally need to slow them down a lot to win. Take risks if necessary to establish a clock because you don't have the tools to shut them down for long and try to get maximum value from your Ceremonious Rejections and Surgical Extractions.
With Ironworks picking up in popularity, I'm considering a white splash for Stony Silence in the sideboard a la Richard Tan , and I'd definitely make the leap if your local metagame is high on Ironworks and Mono-Green Tron.
Game 1 here is about transforming Thing in the Ice and then trying to close the game before they rebuild or burn you out. That's a tough ask in a post-Creeping Chill world, but it's the best we have. A double-Phoenix draw or turn 4 Crackling Drake can steal games too, but that requires them to stumble for at least a turn.
After sideboarding things get a lot easier. Once again, use your Spell Pierces aggressively to stop their enablers and buy time, because you'll be able to handle their creatures with Thing in the Ice and Anger of the Gods and win a long game provided you don't fall into burn range. To this end, I prioritize Creeping Chill and Conflagrate with Surgical Extraction. If you're on a Crackling Drake plan, hitting Stinkweed Imp is a high-value target as well.
I'd wager this is Izzet Phoenix's worst matchup among the most commonly played decks and there isn't a secret plan to turn it around. Protect your life total and interact as much as possible on the early turns so casting Crackling Drake isn't a death sentence. You'll need to end the game by turn 6 at the latest to consistently have a chance.
The one tricky decision in this matchup is whether or not to block with Thing in the Ice. You save some damage but open one of your best creatures to trading for a removal spell. I default to blocking since trading resources is a net positive, but if you have a hand that's high on disruption and low on threats you must value Thing in the Ice more highly.
You're going to kill Eidolon of the Great Revel on sight most of the time, but sometimes you can find a spot to lock them underneath it with some burn in response and an attack in the air. Look out for those spots and capitalize on them.
VS Mono-Red Phoenix
I look at this deck as a better version of Burn, though it's not as bad for Izzet because you can interact with their creatures more readily than Boros Charm and company. They aren't nearly as good at recurring Arclight Phoenix so tagging it with a Lightning Bolt isn't as bad as it typically feels.
Like Burn, protecting your life total is important here, as is killing Eidolon of the Great Revel. Thing in the Ice and Crackling Drake are both difficult to answer for them, so I'm okay playing a longer game even if they're gaining card advantage with Bedlam Reveler, which is why I'm bringing in Surgical Extraction, even though it's virtually useless outside of stopping Arclight Phoenix.
As for Spell Pierce to target their enablers and burn spells, I don't seeing it retaining enough value into turns 3 through 5 to be worth the blowouts when you tag a key Faithless Looting or Tormenting Voice. That's one of the questions I still have to answer moving forward.
VS Izzet Phoenix
As I noted above, the mirror is really about Thing in the Ice. Barring an unanswered double Phoenix draw, the first person to transform Thing is very far ahead, likely enough to finish the game with some Lightning Bolts. To that end I could see some number of Gut Shots still being good, but protecting your Thing in the Ice from removal or Arclight Phoenixes from Surgical Extraction is enough for me to play some counterspells.
VS Hollow One
Another matchup where Thing in the Ice is very important, but a well-timed Lightning Axe can also buy plenty of time for Crackling Drake to come down and end the game. They don't have much disruption for your creatures, but they will take over against your removal in a longer game so once again, turn the corner quickly.
I don't bring in Anger of the Gods because answering the big creatures is much more important than the small. Surgical Extraction helps against the latter and is more impactful than Gut Shot, so it gets the nod as the free spell even though it's far from an all-star here.
Infect breaks the mold of combo decks being matchups where you prefer Monastery Swiftspear to Crackling Drake and that's because the latter is an excellent blocker. It's basically impossible for them to pump a creature large enough to trade for it and if they do, you're up enough cards that you're usually still ahead.
The Infect matchup is another one where interacting early and often is important, but you have to do so wisely. Finding the right time to cast your spells so as to lose the least against their interaction is important, so I typically pick my fights on their end step. Use your poison counter as a resource, because they have virtually no reach. I also like to set up a Thing in the Ice on one counter so I can transform on their turn with one spell, often forcing them to do nothing and letting me untap with an Awoken Horror to start the pressure.
A lot of players bring in Anger of the Gods here since they rely on small creatures, but it's a huge liability against Spell Pierce and often only trades one-for-one anyway. While you do want to trade in this matchup, you have to do so efficiently or they can find an opening and steal a game.
This matchup is rather like Infect, though they have some better tools to play through your removal. Arcbound Ravager is the best card besides Hardened Scales and keeping it off the battlefield is a huge priority. Hangarback Walker can be annoying, especially if you're trying to attack in the air, but without a sacrifice outlet it's often irrelevant.
Thing in the Ice is, unsurprisingly, incredible in this matchup and you'll rarely lose a game in which you transform one because their curve is glutted at two mana so it's difficult for them to re-deploy their threats in a timely fashion.
My biggest suggestion for this matchup is to be aware of what the Hardened Scales player is capable of. They can win from very innocuous battlefields if you're not careful, so play from their side if possible and learn to recognize what their outs are so you can play around them, though you do have a fair number of games that come down to fading a topdeck.
VS Grixis Death's Shadow
This is the matchup I'm least confident in. The card Death's Shadow is very hard to play against with Arclight Phoenix and Lightning Bolt, and nearly all of their disruption is good against you. That said, Thing in the Ice is great at setting up big attacks, which is exactly what you want to be doing against Death's Shadow, killing them in one turn so they don't have time to leverage their Death's Shadow.
Try to set up a turn like that if possible, letting them do the early work of damaging themselves so you can finish them off from ten or thirteen life with some combination of Awoken Horror, Arclight Phoenix, and Lightning Bolt.
If you're unable to do so and you end up in a race against Death's Shadow, then Temur Battle Rage is a problem, which is why I bring in the Dispel. It also protects your creatures from Fatal Push and Dismember, and reliably filling those two roles puts it ahead of Spell Pierce's ability to target discard spells.
You could also bring in Surgical Extraction over the other two Gut Shots, but in this matchup every point matters. They'll often put themselves to one more than a multiple of three against Lightning Bolt, or look to chump an Awoken Horror with Snapcaster Mage, so I actually think it's more functional than Surgical Extraction, which is historically not a card that "counters" Snapcaster Mage.
VS Mono-Green Tron
Tron is essentially a combo deck, and you struggle to win once they resolve anything big, so keep them off the Urzatron for as long as possible and hope to burn them out or counter their first big threat with lethal damage on the battlefield. You're going to kill them on turns 4 to 6 if unabated, so you don't get to buy much time here, and because you do most of your damage in the air, Wurmcoil Engine is often not enough.
You may be tempted to find room for the second Abrade, but it's very underwhelming in a deck that isn't attacking their manabase. On the play it can destroy Expedition Map, which is a great trade, but other than that you're mostly hoping they stumble so you can get an Oblivion Stone or stall a Wurmcoil Engine, neither of which is particularly attractive. Keep in mind that you can force an Oblivion Stone on their turn or your main phase, letting you recur Arclight Phoenixes and attack unabated.
Nothing special to this matchup. Kill their creatures and use your Surgical Extractions well. This is another matchup where playing it from the other side gives very valuable insight for determining what they're capable of and what cards are going to be important in a given game. Storm is great in long games, so you need to find your window to land a threat and take advantage of it.
If the results of the last month are any indication, Izzet Phoenix is going to be staying around for the foreseeable future as one of the better decks in Modern, so get your reps in with or against it now.