I'm at this quest too.
First tried Katrenn, then skipped her to come back later (with the alternative of waiting for her at the west entrance).
But, I got this very same problem with the imp.
And, actually, when people say no AOE spell, they mean it, this includes buff spells as well.
In a fight the 1st thing I do is a +might buff with Xoti.
Reloaded, did not do the buff, and the guards did not aggro.
I could use disintegrate from Serafen though (which is a single target spell).
So, just use weapon, single target, attack (I did not try with AOE weapon attack, this may also trigger the guards) or single target spells.
[Edit] For Katrenn, the automatons do AOE spells. So did her with waiting to the west up to 8PM, clicked her to start conversation, wait the automatons exit the screen, choose attack. But still, do not do any AOE spells (even buff).