Destroy each creature that enters the battlefield under an opponent's controlDestroy all creatures opponents controlLock
How it works
If needed, give Sephiroth deathtouch.
Activate Sephiroth using Walking Ballista's ability by removing a +1/+1 counter from it, dealing 1 damage to a creature an opponent controls and destroying it due to Sephiroth's deathtouch.
When the creature dies, Sephiroth triggers, putting a +1/+1 counter on it.