Activate Phyrexian Altar by sacrificing Phylath, adding 1.
Phylath dies, triggering Nim Deathmantle, causing you to pay 4 to return Phylath from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Phylath enters the battlefield, creating at least four 0/1 Plant creature tokens.
Activate Phyrexian Altar three times by sacrificing three Plants, adding 3.