Activate Altar of Dementia by sacrificing Su-Chi Cave Guard.
When Su-Chi Cave Guard dies, it and Nim Deathmantle triggers.
Resolve the Su-Chi Cave Guard trigger, adding eight C.
Resolve the Nim Deathmantle trigger from step 2, causing you to pay 4 to return Su-Chi Cave Guard from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Resolve the Altar of Dementia ability from step 1, causing target player to mill cards equal to Su-Chi Cave Guard's power.