Fairies are the best damage dealers with Angas Ruby and Issharas Whip.
Green Dragons are very good mana supply. Helps especially when the battle reaches Resurrection mode (only a slowed opponent stack for mana draining left). In combination with Mana/Rage charges this helps extremly with resurrecting losses.
Inquisitors + Gift = unlimited Lev 5 unit resurrection.
Dryads + Gift = unlimited sleep for lev 1-3 units and also a good damage dealer with bless from Inquisitors.
Giants are very good midgame AoE damage dealer and shield (especially against archers using target). Too slow for engame tho.
Archmages absolutely underrated midgame unit. The combination Shield + Target + Giant basically gives an arrow magnet with 1800 Health, that can easily be healed. Also against Karador on Impossible: Griffons (with the -30% leader Reagalia) + Shield(Archmages) + Stone Skin + Target = extreme Tank with unlimited Retaliation that all sorounding troups attack. Especially deadly with Necromancers that damage all units around the Griffon when attacking and so kill their own troops.
Those were the main combat units, for the final fight i used: Paladins, Knights, Ancient Ents, Unicorns and Lake Fairies, avoiding the heavy Focus on magical damage(bad against dragons) of the normal setup combined with damage resistance.
Monks are also exceptionally good against undead, beating archers by far.