Lifegain decks grind out incremental advantage through ETB lifegain triggers (Hinterland Sanctifier, Haliya, Case of the Uneaten Feast) and eventually plan to go over the top with Liliana the Faultless and Ajani Resolute, so Clarion Conqueror stops the activated abilities of their creatures and planeswalkers, including Liliana's hexproof ability and Ajani's loyalty abilities. It does nothing against Lunar Convocation, which is an enchantment, so its draw ability still works. Surrak, Elusive Hunter punishes their removal (Erode) by drawing us cards and can't be countered. We trim Ba Sing Se since this matchup is more about applying pressure before they stabilize than grinding a long game, and cut some Esper Origins since our own lifegain is largely irrelevant against a deck already built to out-grind lifegain.