That's good to know about the enemy switching tactics!
Yes, I use slowing/holding spells (grease, entangle, web, chromatic orb) and spike growth, as well as call lightning, static charge, and cloudburst. Of course, you need to keep your melee folks out of the fray in these cases. Web has been my favorite so far because of the wide paralyzing effect (except for spiders).
I've found it easier to give my tanks the rings of free movement, and have then do the damage with magical weapons if there aren't too many other enemies.
