Scapeshift is an odd card, but a powerful one when combined with Valakut, the Molten Pinnacle. The thing is, it's one of those cards that is only powerful in that specific context and is otherwise pretty useless. You'd never put it into a deck playing a normal gameplan, nor would you want to pick it in a draft.
But the tried and true ramp style of Scapeshift isn't the only way to play the card. As it turns out, the card plays quite nicely with landfall. Even if you don't have a pile of lands on the battlefield, searching for four or more lands is more than enough to generate a significant number of landfall triggers. The only question is what you're going to do with those triggers.
The answer is, of course, kill your opponent.
Steppe Lynx and Plated Geopede come down before Scapeshift, leading to some easy Turn 4 kills if left unchecked. And even if your opponent has blockers, you can tutor for a bunch of fetchlands, force a chump block or two, and reload the next turn with no fuss.
Knight of the Reliquary serves as an honorary creature in this capacity, not getting as significant a bonus from sacrificing lands but compensating by being a much more powerful creature overall while searching for key Valakuts or perhaps the singleton Sejiri Steppe to force through the last few points of damage.
Boom // Bust supplements this new, aggressive gameplan. You can easily target your own Flagstones of Trokair or a fetchland to break the synergy, effectively following up your aggressive creatures with an undercosted Stone Rain, and if you ever get to cast the Bust half, you'll quickly learn why Armageddon is one of my favorite cards in Magic's history. (Hint: It's Bust-ed.)
With this aggro plan, the Valakut kill is secondary. You aren't very good at ramping in this deck, with only Explore to facilitate extra land drops, so Prismatic Omen turning on Valakut on your sixth land is key. It also fixes your mana nicely and allows you to conserve fetchlands to set up big turns if desired, so it's among the few cards tying the two aspects of the deck together.
Like with a typical Scapeshift list, there isn't much room for interaction here, but the versatility makes this list more resilient to the kinds of answers that typically fell Scapeshift: discard spells, counterspells, and anti-shuffle effects like Aven Mindcensor. The fact that you haven't sacrificed much in terms of speed to gain that versatility is a good sign.