Prototype Portal + Dross Scorpion + Yahenni, Undying Partisan + Su-Chi

Prototype Portal

Dross Scorpion

Yahenni, Undying Partisan

Su-Chi

How it works

1. Activate Prototype Portal by paying 4 and tapping it, creating a token copy of Su-Chi.
2. Activate Yahenni by sacrificing Su-Chi.
3. When Su-Chi dies, Su-Chi and Dross Scorpion trigger.
4. Resolve the Su-Chi trigger, adding CCCC.
5. Resolve the Dross Scorpion trigger, untapping Prototype Portal.
6. Resolve the Yahenni ability, giving it indestructible until end of turn.
7. Repeat.