Yes, that's how it goes. Without weakening the boss, you have around 3-4 turns - depending on whether you can disable the boss or not - before he possesses Lohse. If you check the journal, this means you have lost Lohse for good.
You need pretty good builds/tactics to take him down before this happens. Normally in a fight like this, you'd focus on the mobs first, then turn to the boss. But in this particular fight, you have to go hard on the boss.
If you have weakened the boss, though, you don't have to deal with this issue.
Or, another way to beat this is to kill him without letting him transform at all. You just need someone with Initiative high enough to go first, then disable the boss (polymorph, sleep, etc.). Or simply kill him without letting him have his turn, of course.
