Yahenni, Undying Partisan + Agatha's Soul Cauldron + Walking Ballista

Yahenni, Undying Partisan

Agatha's Soul Cauldron

Walking Ballista

How it works

1. Give Yahenni deathtouch, if needed.
2. Activate Yahenni using Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to a creature an opponent controls, and destroying it due to Yahenni's deathtouch.
3. Yahenni triggers, putting a +1/+1 counter on it.
4. Repeat from step 2 as desired.
5. If you can give Yahenni deathtouch indefinitely, you may repeat this loop whenever a creature enters the battlefield under an opponent's control.