Quest Targets respawning. Bosses disappearing

[ibol]

Fennrick,
To my knowledge I do not experience either of these behaviors.
However, I will try to re-create both this weekend.
(I am not saying you are mistaken, I am approaching it from an empirical viewpoint, with a mind to what would cause that.)

And about achievements:
Remember that If you are playing without perma-death, then no achievements will register. One of the caveats to the hardcore.

[Fennrick]

Of course i play with perma death on.

[Kazeto]

I have to say that I had once seen a quest target respawn. It might or might not count, though, seeing as while I had dealt more than 90% of total damage, the target got finished by something else.