still, it's better to choose a Mage as a hero, because if you're lucky and get the "Summon Elementals" spell at level 5, you can summon faster and stronger creatures and, in addition, summon elementals in battle. However, you need to have a lot of mana and enough power to summon more than 100 elementals at once.
Especially against Krell or Thazar, spamming elementals is a good idea. The Armageddon tactic has one significant drawback: a Red Orb will ruin it! Although, it's still worth taking Fire Magic if it's offered, especially for Inferno, except when fighting Flax, as they have too many units that are immune to Armageddon and Berserk, the two most useful spells of the Fire school.