Nim Deathmantle + Ashnod's Altar + Turtle Blimp

Nim Deathmantle

Ashnod's Altar

Turtle Blimp

How it works

1. Cast Turtle Blimp by paying 5.
2. When Turtle Blimp enters, it triggers, creating a 2/2 Mutant creature token.
3. Activate Turtle Blimp's crew ability by tapping the Mutant, causing Turtle Blimp to become a creature until end of turn.
4. Activate Ashnod's Altar twice by sacrificing the Mutant and Turtle Blimp, adding CCCC.
5. When Turtle Blimp dies, Nim Deathmantle triggers, causing you to pay 4 to return Turtle Blimp from your graveyard to the battlefield.
6. Repeat from step 2.