I once did this with 3 SMNs. We kept triggering hellfire way too soon and glitching him -- even on the 2nd set of nails. (He'd literally spawn them and then IMMEDIATELY hellfire before we even had a chance to kill them). After wiping a few times to having too much dps, we started letting our dots fall off Ifrit near the time when nails would spawn. It's silly that we had to do that, but... that's how we managed to win. It was a farm party too and we died a few times to this failsafe before just giving up and letting dots fall. XD