So, I was doing the hydra's cave and when ebon appeared again you couldn't target him.
His icon wasn't there but he could still attack and such. No matter what me and my friends tried we couldn't defeat him.

Same problem here, followed the instructions and ended up with two "Defeat Ebon" instructions at the top of the screen but was unable to target the one Ebon (with either range or melee) that was standing in the same square the whole fight. Often the graphic would show two heads in that square at the same time.

Other issues include when new heads would spawn they would often get flanking attacks for no reason, some heads would do multiple (at one stage I counted six) vengeance attacks after only been hit once.

I now have zero desire to try this fight again and would (after reading up on what others have also posted) put this down as one of the worst designed boss fight I've seen in any MMO I've played.