Blightbeetle + Carrion Feeder + Coveted Falcon

Blightbeetle

Carrion Feeder

Coveted Falcon

How it works

1. Activate Coveted Falcon by paying 1, turning it face up.
2. Coveted Falcon triggers, causing an opponent to gain control of Blightbeetle and causing you to draw a card.
3. Activate Carrion Feeder by sacrificing the undying creature.
4. 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.
5. Resolve the Carrion Feeder ability.
6. Repeat from step 3.

This cunning setup hinges on using Coveted Falcon's face-up trigger to hand your opponent control of your Blightbeetle, locking down their ability to put +1/+1 counters on creatures. With the beetle firmly on their side of the board, your sacrificed undying creature keeps returning to the battlefield completely free of its usual counter penalty. You can then feed that endless cycle into Carrion Feeder to stack up infinite sacrifice triggers, death triggers, and massive +1/+1 counters.