Infinite +1/+1 counters on a creatureInfinite creature ETBInfinite creature LTBInfinite creature sacrifice triggersInfinite death triggers
How it works
Activate Grimgrin by sacrificing the persist creature.
When the persist creature dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it and at least one +1/+1 counter on it due to Tromell.
Remove a -1/-1 counter and a +1/+1 counter from that creature as a state-based action.