To proliferate, you can choose any permanent that has a counter, including ones controlled by opponents
As with last time, we are going to stop the infection in a few ways
First, the active player (or, if it's not an opponent's turn, the next opponent in turn order) chooses which creature they'll exile, then each other opponent in turn order does the same, knowing choices made before their choice
We can play it, make three 2/1 fliers, gain six life, draw three cards, and then immediately gift it to our opponent so they'll have to sacrifice a creature, discard a card, and lose 2 life on their end step
Even though most Doomsday lists dont play Cavern of Souls in the main-deck at the moment, you should always try to counter Doomsday instead of letting them make a pile
But I'm glad they did since Swarmyard Massacre is the best black board sweeper we've seen in years