Cast Triumph of the Hordes by paying ({2} magic symbol) ({G} magic symbol) ({G} magic symbol) , giving creatures you control +1/+1, trample and infect until end of turn.
Cast Windfall by paying ({2} magic symbol) ({U} magic symbol) , causing each player to discard their hands, and draw cards equal to the greatest amount of cards discarded this way.
Razorkin Needlehead triggers for each card your opponents draw.
Resolve the first trigger, causing the opponent to get a poison counter.
If the opponent has ten or more poison counters, they lose the game as a state-based action.
Repeat from step 4 for each remaining trigger, and/or until each opponent has lost the game.