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a) No need for glitches in the guild: summoning elementals is automatically added to the hero's spellbook upon receiving the "Fire Warriors" perk.
b) Mana won't be a problem: 30-40 will be enough (don't forget about "Spiritual Bond" on your own or enemy units). The "Spell Power" stat increases primarily for Necromancers, so there won't be any problems with that.
c) Yes, I agree that a hero with an existing summoning skill is good, but this is just a specific case...
It seems you don't understand much (read the manual:rolleyes:, I mean the manual): when a new blessing is applied, the previous one no longer works. Constantly applying "Dominate" and a blessing won't be very useful: firstly, it requires a lot of mana (it's understandable that Necromancers have a lot of mana, but it's not infinite), it's much more effective to use mass spells at the beginning of the battle, and then "Dominate" and a blessing may be needed.
In any case, the main tactic for Necromancers in the final battles is to prolong the battle.