Activate Fain's second ability by tapping it and removing a counter from a creature you control, creating a Treasure artifact token and a 1/1 Squirrel creature token that enters the battlefield with a +1/+1 counter on it due to Grumgully.
When the Squirrel enters the battlefield, Intruder Alarm triggers, untapping all creatures.
Repeat.
Once you have infinite Treasure and Squirrel tokens, you may activate Fain's other abilities.