Metalwork Colossus + The Ooze + Grinding Station + Master Biomancer

Metalwork Colossus

The Ooze

Grinding Station

Master Biomancer

How it works

1. Cast Metalwork Colossus by paying 0, which enters with at least two +1/+1 counters on it due to Master Biomancer.
2. When Metalwork Colossus enters, Grinding Station triggers, untapping Grinding Station
3. Activate Grinding Station by tapping it and sacrificing Metalwork Colossus.
4. When Metalwork Colossus dies, The Ooze triggers, creating a Mutagen token for each +1/+1 counter Metalwork Colossus had.
5. When the Mutagens enter, Grinding Station triggers that many times, untapping Grinding Station
6. Resolve the Grinding Station ability from step 3.
7. Activate Metalwork Colossus by sacrificing two Mutagens, returning it from your graveyard to your hand.
8. Repeat.