Also, one of the best abilities of pet dragon for mage hero is Ligthning Ball, because it's percentage based and hits where it would hurt most. It is the second ability I max, after treasure hunter. BTW I use mostly Invis II, because it's kind of sweet spot between duration and mana spent, but it depends on enemies and their positioning. Amount of Mana recovered by EGD depends on damage (1 mana per 100 HP I think) and sometimes you get less than you'd like to - when enemies scatter or have high magical resistance. Another one known and good strategy against tough guys is paladin+phantom, but I had no phantom at level 40, and I think that paladins are better against bosses and physical damagers.
In KBTL there was unlimited rage and mana thanks to inquisitors and ice spirit, so almost every big fight was easy (with gift or resurrection), even if tediously long, but in KBAP it's impossible. Either EGD or Calm Rage (rage->mana) are way to go for mana, and lower level creatures and resurrection if you want to bring more troops. Unfortunately with nerfed/changed Anga's Ruby (Ring of Arianna boosts pirates instead of ladies) there's no universally good low levels, but I prefer priests and inquisitors, and beholders too (beholders suck against magic resistant but are perfect for aggro
). It depends on items and availability, human troops are also good and durable. And dryads are still very good with their summons and lullaby. Ah, and demons (meatshield/avenger angel carrier) can be resurrected, even summoned ones, and I prefer to summon them than to pay and bring them to the fight. Besides, demons lower morale of most other races if you keep them in the main army.