Cathars' Crusade, Nest of Scarabs, and Devoted Druid

Prerequisites

• Cathars' Crusade on the battlefield
• Nest of Scarabs on the battlefield
• Devoted Druid on the battlefield
• Devoted Druid does not have summoning sickness
• Devoted Druid does not have a -1/-1 counter on it

Steps

1. Activate Devoted Druid's first ability by tapping it, adding
2. Activate Devoted Druid's second ability by putting a -1/-1 counter on it
3. Nest of Scarabs triggers, creating a 1/1 Insect creature token
4. The Insect enters the battlefield, triggering Cathars' Crusade, putting a +1/+1 counter on each creature you control
5. Resolve the Devoted Druid ability from step 2, untapping it
6. Repeat

Results

• Infinite creature tokens
• Infinite ETB
• Infinite green mana
• Infinite +1/+1 counters