Doomsday is a combo deck that wins by casting Doomsday to build a 5-card library and then winning through Thassa's Oracle, so your primary goal is to disrupt their combo before it resolves while pressuring their life total. Leyline of the Void is your best weapon — it exiles cards as they're drawn through the Doomsday pile, making Thassa's Oracle unable to win since the library won't be empty. Containment Priest stops Thassa's Oracle from entering the battlefield via its own triggered ability while also being a relevant clock. Red Elemental Blast counters Force of Will and can hit Murktide Regent. Keep up your hand disruption with Thoughtseize and Cabal Therapy to strip Doomsday or Force of Will before they can go off; Ajani, Nacatl Pariah and the rest of the aggressive package apply enough pressure that they must combo quickly into your interaction.