Agatha's Soul Cauldron + Wildwood Mentor + Krark-Clan Ironworks + Thopter Squadron

Agatha's Soul Cauldron

Wildwood Mentor

Krark-Clan Ironworks

Thopter Squadron

How it works

1. Activate Wildwood Mentor using Thopter Squadron's first ability by paying 1 and removing a +1/+1 counter from it, creating a 1/1 Thopter artifact creature token.
2. When the Thopter enters the battlefield, Wildwood Mentor's first ability triggers, putting a +1/+1 counter on itself.
3. Activate Krark-Clan Ironworks by sacrificing the Thopter, adding CC.
4. Repeat.
5. Once you have infinite mana, you may forego sacrificing the Thopters.
6. Once you have infinite creature tokens, you may activate Wildwood Mentor using Thopter Squadron's last ability any number of times.

By exiling Thopter Squadron under Agatha's Soul Cauldron, you grant its activated abilities to any creature you control holding a +1/+1 counter—in this case, Wildwood Mentor. You can then pay {1} to remove a counter from that creature and spawn a 1/1 Thopter token, which immediately triggers Wildwood Mentor's own ability to replace the spent counter while generating extra growth. Sacrificing that newly created Thopter to Krark-Clan Ironworks yields two colorless mana, leaving you with a surplus to fuel the loop infinitely.