Workhorse + Nim Deathmantle + Ashnod's Altar + Warstorm Surge

Workhorse

Nim Deathmantle

Ashnod's Altar

Warstorm Surge

How it works

1. Activate Workhorse by removing four +1/+1 counters, adding CCCC.
2. Activate Ashnod's Altar by sacrificing Workhorse, adding CC.
3. Workhorse dies, triggering Nim Deathmantle, causing you to pay 4 to return Workhorse from your graveyard to the battlefield, and attach Nim Deathmantle to it.
4. Workhorse enters the battlefield with four +1/+1 counters on it.
5. Warstorm Surge triggers, dealing 4 damages to any target.
6. Repeat.

By stripping away Workhorse's four +1/+1 counters for mana and then feeding the creature itself to Ashnod's Altar, you easily generate the exact four mana needed to satisfy Nim Deathmantle's resurrection trigger whenever Workhorse hits the graveyard. This infinite loop cleanly fuels itself while the passive presence of Warstorm Surge weaponizes every single rebirth, raining down lethal damage equal to Workhorse's power upon any target you choose with each iteration.