I'm doing a fate solo for my animus, it's difficult especially as a non healer class (monk) but between using second wind, bloodbath, potions, a tank chocobo and kiting I can sustain. A random tank joins half way through the fight, great right? No. He takes all the enmity, loses all his hp and runs away with the fate boss at 5% hp and resets it with just a minute left on the timer. This isn't fair on me or any other player who comes close to reaping a reward for very challenging gameplay only to have it completely ruined by someone else. I didn't agree to be in a party with this person, i don't feel my game experience should be so directly and negatively affected by them.
Please consider a different system for handling fates like this, I'd suggest making enemies auto switch to the next target highest on their enmity list rather than resetting their state and recovering their full hp.