Blightbeetle + Blasting Station + Switcheroo

Blightbeetle

Blasting Station

Switcheroo

How it works

1. Donate Blightbeetle to an opponent
2. Activate Blasting Station 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. When the undying creature enters, Blasting Station triggers, untapping Blasting Station.
5. Resolve the Blasting Station ability from step 2.
6. Repeat from step 2.

To pull off this destructive loop, you first need to use Switcheroo to hand your opponent a Blightbeetle, locking down their board so creatures can never receive +1/+1 counters. With that passive restriction firmly in place, you can feed an undying creature to Blasting Station over and over again; because Blightbeetle prevents counters from being placed, the creature refuses to stay dead and returns without the counter that would normally shut down its recursion. Every time the creature loops back from the graveyard, Blasting Station untaps itself to fuel the next sacrifice, turning an infinite cycle of deaths into endless damage.