Necrotic Ooze, Devoted Druid, and Grim Poppet

Prerequisites

• Necrotic Ooze on the battlefield
• Devoted Druid in the graveyard
• Grim Poppet in the graveyard
• Necrotic Ooze does not have summoning sickness
• There is at least one additional creature on the battlefield

Steps

1. Activate Necrotic Ooze using Devoted Druid's first ability by tapping it, adding
2. Activate Necrotic Ooze using Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it
3. Activate Necrotic Ooze using Grim Poppet's ability by removing a -1/-1 counter from it, targeting any other creature
4. Holding priority, activate Necrotic Ooze using Devoted Druid's first ability by tapping it, adding
5. Repeat from step 2

Results

• Infinite -1/-1 counters
• Infinite green mana