So now that my cursor is a lightning bolt, I’m going to click on the trolls selecting them as my target. After the computer announces there is combat, a combat window appears. 

I'll arrange my units so that my magicians are lower in the stack. You can rearrange your armies by dragging them up and down in the list or using the Group button to group like units together. This represents the units order of combat.

When both players are done rearranging, (neutral units don't get to rearrange) hit the Do Battle button.

I managed to kill one troll.  Movement can now continue. To give my legions a better chance, I'm going to advance some Northstar Cavalry.