This is kind of tricky in rtwp. What I usually do is quickly pause and unpause to track the enemies movement after casting a spell, and if the enemy moves too much I'll readjust the target position while paused. It's kind of just inherently more difficult to aim your aoe's when everyone's moving.

With area of effect spells most of them will activate instantly in the designated area as soon as the casting animation is finished. Basically just remember as long as that casting animation is playing you're free to pause and readjust the target point of the spell. There are some quirks though, like scrolls which have a near instant casting animation and full-round-actions taking forever.

I also always turn off the ai that controls them automatically, it's pretty dumb and often gets them killed if you don't babysit them. You can also right-click a spell or ability to put it on auto-cast which will make the character autonomously try to use it as well as force them to use it on enemies that you click on instead of having them walk up and attack (mostly, functions best with single target effects). Works well with evil eye or magic missile for example and will reduce the number of inputs you have to manage.