Mystical Mirror Water Cave only certain class combos can do it - it requires the player to one shot all mobs of the slug variety or they become invulnerable. Once invulnerable nothing can kill them. This is clearly a bug when all classes (104 level) cannot clear a level 80 instance due to mechanics. Look at the difficulty of all the other instances, not one issue there. Yet you have this insane mob that cannot be killed unless it is one shot killed. Even then, the way the mobs are positioned far apart and chain as a group, chances are one or 2 will get off an invulnerable and there's nothing to be done from then on. Do I really have to go scout to one shot these mobs? I shouldn't have to. It is bugged as bugged can get, you know it, I know it and so does the player base yet they put up with it. Just because your logs say players have and do finish it doesn't mean there isn't a problem.
Solution - tame the invulnerable spell the slugs do, at the moment they refresh it even when there is still 10-15 seconds left on the old one. Let the old one run out at least - either that or do a damage absorb cap on the bubble so we can at least kill it.