I think killing Zhalk in 15 turns is totally feasible.
Killing the other 2 cambion dudes might be a challange, but killing one of them is probably not too hard.
Very doable - if you stack the nautiloid tanks in the corridor outside the helm, leave Us there and throw grease bottles on the ground to slow the cambions when they show up. Then beat on Zhalk and use the tanks in the room to finish him and the mindflayer. Takes about 10 turns total but you do need the mindflayer to not miss very often...