Donatello, the Brains + Animation Module + Mutagen Man, Living Ooze + Berta, Wise Extrapolator

Donatello, the Brains

Animation Module

Mutagen Man, Living Ooze

Berta, Wise Extrapolator

How it works

1. Activate a Mutagen by paying 0 due to Mutagen Man, tapping and sacrificing it, putting a +1/+1 counter on Berta.
2. Berta and Animation Module trigger.
3. Resolve the Berta trigger, adding 1.
4. Resolve the Animation Module trigger, causing you to pay 1 to create a 1/1 Servo artifact creature token and a Mutagen token.
5. Repeat.