Lifegain decks like this grind out incremental value through cards like Enlightened Confidant, Amalia Benavides Aguirre, and Moseo, Vein's New Dean returning creatures from the graveyard, so Rest in Peace is a huge blowout, exiling their graveyard and permanently shutting off Moseo's recursion and Case of the Uneaten Feast's solved ability. Chainsaw is a cheap one-shot answer: it deals 3 damage to a creature when it enters, which kills most of their small lifegain creatures, and it can then be equipped later, gaining +X/+0 for each rev counter it collects as creatures die. Doc Ock's Tentacles and Basilisk Collar are cut as too slow against a grindy lifegain shell, and Raubahn is the least essential of our creatures here, so it makes room for the sideboard cards. Prioritize applying early pressure before their engine of 1-life-per-creature snowballs out of control.