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Generally, only a mage can be better than a mage... As they say, if you're a warrior and you don't have a Ward of Negation or a Cloak of Negation, it's best to engage a good mage only with forces that outnumber him by at least three times.
So, the ideal build... 4 elements, intelligence, wisdom, spell power, and the last one of your choice... for example, I like diplomacy... it often happened that troops joined me, for example, 54 Titans when I myself had only about 30, and often even for free.
My favorite spells are:
water... blessing, clone, prayer, healing, forgetfulness, weakness.
fire... curse, berserk, inferno, execution, blindness, frenzy.
earth... portal city, slow, antimagic, restoration, explosion.
air... haste, destructive ray, chain lightning, counterspell, fly, dimension door.
The most effective combinations of curses:
slow and curse... (long-term against a warrior hero)
blindness and repeatedly used destructive ray (until the defense drops to zero) against very powerful monsters.
slow and forgetfulness... well, it's obvious.
Berserk... it's effective on its own, and incredibly effective against giant armies at the expert level. For example, such a tactic... cast antimagic on archangels... (you can do without it if you have Titans or Phoenixes)... and wait until you are surrounded... then cast berserk and run as far as possible... or first, disable the strongest ones for a while...
Prayer... the best spell among the blessing spells, and the most frequently used is haste...
Clone... a cheating spell if combined with archangels... I practically become invincible as long as I have mana (provided there are enough archangels... for example... against a hundred bears, 15-20 archangels are enough for me, after dividing them into two groups... Thanks to this spell, I completed one map where I couldn't defeat the computer opponent... after probably the hundredth attack, I fully mastered the tactics of working with clones and won decisively... and all the fuss was because of the Titans... berserk didn't work on them, in this tactic it is convenient to direct the main group of archangels at the Titans and resurrect the fallen with clones)
Frenzy... I use it carefully... in case the monster is very powerful and won't retaliate, and I need to kill it as quickly as possible.
Blessing... a great spell for monsters like Titans, where the damage range is large... 30-60... the difference is immediately noticeable.
Counterspell... it is convenient to use on those who are surrounded or are likely to be attacked constantly.
And finally, healing is the best way to remove all negative effects and heal your troops.
P.S. a mage must have "meat" in his squad that needs to be put in harm's way... otherwise, not much will come of it.