-AoE-Spells need items to be REALLY imba (1-hit-kill), so keep hunting and nuking the AoE-Casters from early on. You gain money, they spend money on porting and staying alive.
-Dispels: Maybe against some heroes. Against others they are totally useless, so it is difficult to say (and even more difficult to balance). Still, Dispels need many points to be effective (rather low cooldown and high summon dmg), so you cannot spend points on something else... which hurts, since the other spells also have their uses (TC got 4 great spells apart from Dispel
)-Obelisks: are meant to be imba. Hey, that's what the game is about, so heck, why not?
-Bane: Get Str., Regen and Evoc items, Caustic breath, Acid Sheath and some armor. Melee units wont deal damage due to Sheath, Ranged dont have enough damage to seriously do something (even Tact when you have armor) and Casters wont, too(Charge+Breath a few times)...
-Chill Sacrifice: Maybe a bit weak, and IS's other skills are way more useful, but it can be annoying if the enemy just refreshes his mana and a few lost hp that way, while you are always below 100 (at least I used to be as some heroes).
-You are right on some of the features
But later one Tomes of Power can be used to gain levels faster, and skill points through that... maybe not the best solution, but as long as you dont spend crystal on Skill Points[I dont wanna say Crystal on Skill points is bad, but usually it is too expensive to do in a challenging game, and you've got other priorities (damage and int for me, usually)]
