Activate Phyrexian Altar by sacrificing Mitotic Slime, adding 1.
Mitotic Slime dies, triggering itself and Nim Deathmantle.
Resolve the Mitotic Slime trigger, creating two 2/2 Ooze creature tokens.
Activate Phyrexian Altar by sacrificing an Ooze, adding 1.
The Ooze dies, creating two 1/1 Ooze creature tokens.
Activate Phyrexian Altar twice by sacrificing two Oozes, adding 2.
Resolve the Nim Deathmantle trigger from step 2, causing you to pay 4 to return Mitotic Slime from your graveyard to the battlefield, and attach Nim Deathmantle to it.