Humanbreds should only be used when you need lots of fire resistant troops but haven't researched Fire Fend at Ecnhantment level, umm, 5. (The
Grimoire is great help!)
True-blooded abysians are much, much better.
Great spells to look for are Phoenix Power and Summer Lions at Conjuration and many spells at Evocation. Also, if you can research Alteration for Phoenix Pyre.
Your mages have good hitpoints, so you can put them forth. And you should, because they have hard time hitting anything... Incinerate is a great spell if you fight against small elite armies, and if your enemy uses big fodder armies your mages can hit something. Also, with Flame Eruption researched your mages are awesome even in close combat.
And use Slayers those Slayers! Some good items you might want to give to them are: Sceptre of Authority or dragon helmet (both shoots fire bolts) for beginning and those nasty staves casting Incinerate and Fireball later if you have the gems and/ or Dwarven Hammers. Other good items are those two Contracts, other gives imps in battle and other free Devils for your armies.
In short: research Conjuration and Summer lions for stronger armies, Evocation for more firepower (
) to help your armies and Construction for goodies. Alteration is optional, but there are some spells that really help you out. If you are playing against humans you need protecting Domes, and for that research Enchantment. Also, if you for some reason want to use Astral magic of your Warlocks research Thaumaturgy. One of the reasons for that would be a SC with not-so-good magic resistance.
Propably osmeone else has posted something better by now, but anyway, here you go...