I agree Helplessness is quite useful and not just for few case, I use it a lot to dispose more quickly a dangerous stack.
I also agree that often Weakness isn't interesting in general but with the mass effect there are cases where you won't be able to avoid get hit by multiple enemy units and in those case the mass spell is huge.
Pygmy has a similar use than Helplessness but in different case. I generally prefer Helplessness but when I'm sure I'll get a rude counter attack from the unit attacked.
Invisibility is an excellent spell for few special tactics and I use it sometimes. My standard use case is 2 fast close fighter stack: Throw them in front, both attack then cast invisibility on one of the two. The other unit get all hits. Second turn I repeat and before end of turn I time back the unit that received all hits. That's often quite good with two fast and strong close fighting units with no retaliation and much more efficient than phantom for a similar use case. But that's juts a case there's many more use case for invisibility.
I agree that Healing is cool during the first part of the game. but I see myself use it even in middlegame.
Light of life isn't great but isn't that bad because it's a mass heal that heal any units but undead and demons. Also it has use cases against undead army and when your army has units with high life. Take care that the Healer skill can increase its power of 25%. And an option could be to mix its use with high life units and Peacefulness, never tried.
I don't see much use of:
- Last Hero no idea what use can have this spell,
- Peacefulness (perhaps combined with some Light of life but I doubt),
- non mass Weakness,
- Shroud it should be a sort of mass effect on enemy army,
- Kamikaze which is just a complicate attack spell,
- Summon Phoenix (too much mana, better use Phantom),
- Book of Evil is funny... until random hurt you... then you start find it not that funny
- Greasy Mist if you aren't a mage with the possibility to cast 2 spell in a turn,
- Plague because its effect is too invisible,
- Poison Skull because in general poison effect suck because long term effect are useless.
- Berserk because the random doesn't worth it it break too much any attempt of tactic
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EDIT: Now I'm thinking of it Peacefulness could have some use with an Ancient Ent, its main attack is a special power certainly not changed by the spell and its close attack is so strong that even reduced it would be quite good, 1500 life isn't that bad.
EDIT2: Still about Peacefulness, if you have ton of mana combined with Sacrifice... well ok then better use time back.