Mairsil, the Pretender + Endling + Blood Pet + Crystalline Crawler

Mairsil, the Pretender

Endling

Blood Pet

Crystalline Crawler

How it works

1. Activate Mairsil using Endling's third ability by paying , giving Mairsil undying until end of turn.
2. Activate Mairsil using Blood Pet's ability by sacrificing it, adding .
3. Mairsil dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
4. Activate Mairsil using Crystalline Crawler's first ability by removing a +1/+1 counter from it, adding one mana of any color.
5. Repeat.