Throw them on your front lines in direct contest with your enemy and they will hack them down to size. Warlocks like to hang about the library where they will research magical spells. Once all the spells are researched they will begin to upgrade them too. Warlocks also develop a powerful Fireball spell as they advance in level which is good as a support attack for your front line fighters. The Troll is a builder of traps and doors who works in your workshop.
Trolls are excellent at destroying enemy traps too, and they'll even put up a good fight.
It's your job to be ready for either one or both of these possibilities. A clever dungeon design littered with heinous traps is your best defense. The Healing, Turncoat, and Chicken spells worked for me in these cases. One blemish in the overall mission design was that too many levels had puzzle-like solutions to them. While in possession of a creature, you can gather a small army that will follow you as you lead them in an attack.