Blightbeetle + Fateful Handoff + Phantom Train

Blightbeetle

Fateful Handoff

Phantom Train

How it works

1. Donate Blightbeetle to an opponent
2. Activate Phantom Train 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 Phantom Train ability.
5. Repeat from step 2.

This clever engine turns your opponent's own defenses against them by using Blightbeetle's passive static ability to forbid opponents' creatures from ever receiving +1/+1 counters. With that restriction locked in, Phantom Train can repeatedly devour your creatures for fuel and grow indefinitely without its cost exhausting your loop potential. To get the ball rolling, Fateful Handoff conveniently hands over ownership of Blightbeetle to the other side of the table so its counter-suppression applies precisely where you need it.