Triskelion in exile (exiled by Agatha's Soul Cauldron).
Sengir and Agatha's Soul Cauldron on the battlefield.
Sengir has at least one +1/+1 counter on it.
You have a way to give Sengir deathtouch.
Steps
Give Sengir deathtouch, if needed.
Activate Sengir using Triskelion's ability by removing a +1/+1 counter from it, dealing 1 damage to any creature an opponent controls, and killing it due to Sengir's deathtouch.
When the creature dies, Sengir's first ability triggers, putting two +1/+1 counters on it.
Repeat from step 2 as desired.
Notes
If you have a way to give Sengir deathtouch indefinitely, you can repeat this loop whenever a creature enters the battlefield under an opponent's control.
Results
Destroy all creatures that enter the battlefield under an opponent's control.