Triskelion has two or more +1/+1 counters on it, activate
Triskelion by removing a +1/+1 counter from it, dealing 1 damage to any other target.
Repeat step 1 until
Triskelion has one +1/+1 counter on it.
Activate
Triskelion by removing a +1/+1 counter from it, dealing 1 damage to itself.
Triskelion dies as a state-based action due to having 0 toughness. It's persist ability triggers, returning it from your graveyard to the battlefield with two +1/+1 counters on it due to its ability.