Lowering the amount of mana the player has would be a good start. 10 per skillpoint sounds good for me.
Another idea that i had would be implementing an cooldown. 10 seconds for low stuff like heal, 30 for necromancie and stuff like the blizard and 1 minute for high end stuff like the fireball and firewall, just so you cant spam them.
The main problem i see with them is, whit troops like daemons and golems. Shields could look a little bit odd on them if you ask me.
For the rest, it should be quite easy, for inspiration i would recommend this.
And what about invisible shields? So it looks like they don't have shields but actually they do. Just like the invisible weapons they use.