No, in general they do not respawn. There are a couple places where you can trigger opponents to respawn, and another where you can avoid doing something and let opponents keep coming through some rifts.
In Maxos temple, in the mirror room there is a barrel that spawns opponents when clicked. In Keara's flying fortress trying to open the door to the inner headquarters without correctly solving the statue puzzle will spawn opponents (this is the best place to farm experience, if you wish to do so). In the ministry in Aleroth opponents will keep coming if you fail to protect Zandalor.


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