With all cards in play, cast another creature.
Cathars' Crusade's ability will trigger, put a +1/+1 counter on Scurry Oak
Scurry Oak's ability will trigger, creating a 1/1 token
Cathars' Crusade's ability will trigger again.
Repeat the process for infinite tokens and infinite damage.
Impact Tremors will deal 1 damage for each token that enters the battlefield