He died during a boss battle because the Ruin Sentinels pushed me down to the bottom section and left him behind up top, where two of them killed him. Is there anyway to resurrect him, because I died afterwards too!
She is not a live NPC and doesn't need to be resurrected. You can simply summon her again while you're in human form. You can summon these NPC phantoms for an infinite number of times until the boss is defeated.
Resurrecting NPCs with tombstones is only possible with those who talk to you, so that you don't miss out on their services if you killed them (like merchants).