I did a whole writeup on this:

https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=754044

And then felt foolish because, if you get up the ramps early enough and far enough, there's a good chance Zhalk and the cambions won't follow you, but will instead rage at you from the ground floor while you plink them full of arrows/cantrips. So I had several grand techniques, but 'the combat AI won't path right' is more than enough most times.