Pentavus in exile (Pentavus 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 Pentavus's first ability by paying ({1} magic symbol) and removing a +1/+1 counter from it, creating a 1/1 Pentavite artifact creature token.
When the Pentavite enters the battlefield, Wildwood Mentor's first ability triggers, putting a +1/+1 counter on itself.
Activate Phyrexian Altar by sacrificing the Pentavite, adding ({1} magic symbol) .