Prototype Portal + Dross Scorpion + Grinding Station + Su-Chi

Prototype Portal

Dross Scorpion

Grinding Station

Su-Chi

How it works

1. Activate Prototype Portal by paying 4 and tapping it, creating a token copy of Su-Chi.
2. When the token Su-Chi enters, Grinding Station triggers, untapping itself.
3. Activate Grinding Station by sacrificing Su-Chi and tapping it.
4. When Su-Chi dies, Su-Chi and Dross Scorpion trigger.
5. Resolve the Su-Chi trigger, adding CCCC.
6. Resolve the Dross Scorpion trigger, untapping Prototype Portal.
7. Resolve the Grinding Station ability, causing target player to mill three cards.
8. Repeat.