Mitotic Slime dies, triggering itself and Nim Deathmantle.
Resolve the Mitotic Slime trigger, creating two 2/2 Ooze creature tokens.
Activate Ashnod's Altar by sacrificing an Ooze token, adding ({C} magic symbol) ({C} magic symbol) .
The Ooze token dies, creating a 1/1 Ooze creature token.
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Mitotic Slime from your graveyard to the battlefield, and attach Nim Deathmantle to it.