Ulasht in exile (exiled by Agatha's Soul Cauldron).
All other permanents on the battlefield.
Wildwood Mentor has at least one +1/+1 counter on it.
({1} magic symbol) available.
Steps
Activate Wildwood Mentor using Ulasht's ability by paying ({1} magic symbol) and removing a +1/+1 counter from it, creating a 1/1 Saproling creature token.
When the Saproling enters the battlefield, Wildwood Mentor's first ability triggers, putting a +1/+1 counter on itself.
Activate Ashnod's Altar by sacrificing the Saproling, adding ({C} magic symbol) ({C} magic symbol) .
Repeat.
Once you have infinite mana, you may forego sacrificing the Saprolings.