Mired Deep - Story progression quest bug [Charr]
I was having the same annoying problem, too. I could get him down to 25% then he would go invulnerable and swim back to his spawn point— unable to touch him again until he was 100% health, again.
But I was able to beat the boss by pulling him away from the shipwreck and dancing around him.
PS Nobody in the crew was fighting the boss, just me. As if they were just there to watch…
I do think that the Skales down in the lake on this quest are way too hard to defeat given the limited underwater weapons available to you at this point in the game. They seem very weak compared to your standard weapons, and the enemies are able to regenerate.
In particular, the Skale Tyrant is a pain. It was only really by chance I managed to defeat him- the first time around all of my NPC party were killed, so I had to sneak into the lake, revive them all, defeat each individual Skale one by one and then gang on the Tyrant. The way I defeated him may have been a glitch in itself, as he began after a few seconds to flee away from my party and I, and ended up circled by us. We took shots at him with our spears until he died, but it took a good few minutes due to how little damage we did and how often he regenerated his health- yet he did not retaliate. I think this mission does need a little tweaking.
I think the key may be to take out the non-elite skale, and kill them as they respawn. They seem not only to regenerate but to heal/protect each other.