Thanks for the review!
About the abilities - you do want to pay for them as less as possible, yet I do sometimes use Soul Infusion on my ghost(s) just to make enemy backline busy on the get go. Another trick is to make your melee units "flank" into the enemy backline with Mark On Death.
Also I would suggest you not to spend all of your mana - third of your mana is added to your win bonus, so the less you spend - the more you get. Moreover, 250 mana kept to the side could help you reanimate your forces and win the one battle you lost.
Viewing post in Rise of the Skeleton King comments
Oh, that's actually interesting. I didn't realize there's a bonus for not spending all mana. I only realized active skills are useless - and later on they're even worse, with the obvious exception of the revive.
Yeah, I struggled here also because it seems like later battles are... way too insane. But maybe it's doable and you have to know that you get bonus for not spending all mana (which I didn't notice at all).