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.
Deal combat damage to an opponent using Withengar Unbound, causing them to get at least ten poison counters.
The opponent loses the game as a state-based action due to having ten or more poison counters.
Withengar Unbound triggers, putting thirteen +1/+1 counters on it.
Activate Withengar Unbound using Walking Ballista's second ability by removing a +1/+1 counter from it, giving an opponent a poison counter.
Repeat step 5 until the opponent loses the game as a state-based action due to having ten or more poison counters.