Walking Ballista in exile (exiled by Agatha's Soul Cauldron).
Garza Zol and Agatha's Soul Cauldron on the battlefield.
Garza Zol has at least one +1/+1 counter on it.
You have a way to give Garza Zol deathtouch.
Steps
Give Garza Zol deathtouch, if needed.
Activate Garza Zol using Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to a creature an opponent controls.
The creature dies as a state-based action due to Garza Zol's deathtouch.
Garza Zol's first ability triggers, putting a +1/+1 counter on itself.
Repeat from step 2.
You may also repeat this combo whenever a creature enters the battlefield under an opponent's control if you can give Garza Zol deathtouch indefinitely.
Results
Destroy all creatures opponents control.
Destroy any creature that enters the battlefield under an opponent's control.