Splinter Twin, Barrenton Medic, and Cathars' Crusade

Prerequisites

• Splinter Twin on the battlefield
• Barrenton Medic on the battlefield
• Cathars' Crusade on the battlefield
• Splinter Twin attached to Barrenton Medic
• Barrenton Medic does not have summoning sickness

Steps

1. Activate Barrenton Medic by tapping it, creating a token copy of it with haste
2. When the Barrenton Medic token enters the battlefield, Cathars' Crusade triggers, putting a +1/+1 counter on each creature you control
3. Activate the nontoken Barrenton Medic's second ability by putting a -1/-1 counter on it, untapping it
4. Repeat

Results

• Infinite creature tokens with haste
• Infinite ETB
• Infinite +1/+1 counters