Blightbeetle + Bartolomé del Presidio + Switcheroo

Blightbeetle

Bartolomé del Presidio

Switcheroo

How it works

1. Donate Blightbeetle to an opponent
2. Activate Bartolomé del Presidio by sacrificing the undying creature.
3. When the undying creature dies, its undying ability triggers, returning itself from your graveyard to the battlefield without a +1/+1 counter on it due to Blightbeetle.
4. Resolve the Bartolomé del Presidio ability.
5. Repeat from step 2.

To pull off this loop, you first need to use Switcheroo to hand Blightbeetle over to your opponent, where its static ability shines: because creatures your opponents control can't have +1/+1 counters placed on them, any undying creature you sacrifice to Bartolomé del Presidio will stubbornly refuse to pick up that pesky counter when it returns from the grave. This clever trick lets you infinitely sacrifice and recur your creature right back to the battlefield, feeding Bartolomé del Presidio endless +1/+1 counters while generating boundless death, sacrifice, entry, and leave-the-battlefield triggers.