Donatello, the Brains + Animation Module + Mutagen Man, Living Ooze + Crystalline Crawler

Donatello, the Brains

Animation Module

Mutagen Man, Living Ooze

Crystalline Crawler

How it works

1. Activate a Mutagen by paying 0 due to Mutagen Man, tapping and sacrificing it, putting a +1/+1 counter on Crystalline Crawler.
2. Animation Module triggers. Holding priority, activate Crystalline Crawler's first ability by removing a +1/+1 counter from it, adding 1.
3. Resolve the Animation Module trigger, causing you to pay 1 to create a 1/1 Servo artifact creature token and a Mutagen token.
4. Repeat.