Blightbeetle + Goblin Bombardment + Switcheroo

Blightbeetle

Goblin Bombardment

Switcheroo

How it works

1. Donate Blightbeetle to an opponent
2. Activate Goblin Bombardment 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 Goblin Bombardment ability.
5. Repeat from step 2.

This clever loop turns an opponent's defense against them by using Switcheroo to hand over a Blightbeetle, locking down their ability to place +1/+1 counters on their creatures. With that static prevention active, any creature you control with undying that dies will continually return to the battlefield without ever accumulating the counter that would normally stop it from coming back again. You can then feed these endless resurrections straight into Goblin Bombardment, sacrificing the creature to machine-gun down any target with infinite damage.