Activate Krark-Clan Ironworks by sacrificing Su-Chi Cave Guard, adding CC.
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.