You need to destroy all magic armor to be able to freeze them. Most effects need the specific armor to be destroyed first, before they can be applied.
After Act 1 you will be able to respec as much as you like, sou you can always adjust your memory points, if you need more.
The big issue with skills like Earth Quake: They deal magic damage, but the knockdown effect checks with physical armor. Can work with a mainly physical team, tough teach all melees battlestomp and battering ram is an easier way to keep them down.
