Sometimes when I'm in a battle it wont let me click who I want to use the spell on. It gets me frustrated, especially when there is only one monster, or whatever their called, left, so I have to pass the rest of the battle or flee. I'm not sure if this is my mouse or what.
If you're having trouble targeting a creature, turn on your 2d Combat Display
Press Escape to bring up the Options Menu Fourth tab from the left: Gameplay Options 2d Combat Display: on Then you only have to click the piece of parchment at the top of the window that shows their health etc to target them.