He doesnt need HIPS. IF a DM posses him, in combat, the party will die. There isn't an IF, there.
If a DM posseses him the party wont necessarily die. You, as a DM, have the power to decide what the story will be with a possesed Zelaph. If you want to have a story where you generate a party-wipe; which ends roleplay; then it will most likely occur given the arsenal the creature posseses. There's ways to handle Zelaph with a party of less strength for it to remain a good experience without resulting to a party wipe. That's where shift and focus comes on storytelling - for DM's or players, PvP or (dm)PvM which causes an onslaught often ends roleplay, and does not further it, causing frustration more often than not. Speaking for the multiple times I've handled Zelaph, I've never party wipes, hells; sometimes I can't even emote and the dragon is dead with massive parties.