Druid's Call + Cathars' Crusade + Triskelion + Pir, Imaginative Rascal

Druid's Call

Cathars' Crusade

Triskelion

Pir, Imaginative Rascal

How it works

1. Activate Triskelion by removing a +1/+1 counter from it, dealing 1 damage to itself.
2. Druid's Call triggers, creating a 1/1 Squirrel creature token.
3. When the Squirrel enters the battlefield, Cathars' Crusade triggers, putting two +1/+1 counters on each creature you control.
4. Repeat.
5. During the next player's turn, you may remove all of the +1/+1 counters from Triskelion to deal infinite damage to any target(s).

This clever loop spins up infinitely by turning Triskelion into its own target, shooting itself for 1 damage to trigger Druid's Call and churn out a 1/1 green Squirrel creature token. Every time one of those tokens enters the battlefield, Cathars' Crusade triggers to buff your entire board, supercharged by Pir, Imaginative Rascal which tacks an extra counter onto everything for explosive growth. Since Pir, Imaginative Rascal ensures those counters pile up faster than Triskelion uses them up, you can build up an arbitrarily massive swarm of tokens and deal infinite damage to any target.