And people think way too much about damage
A good army consists of a tank, damage dealers, and support. 1 Tank unit (30% phys resistance) + 2 damage dealer units with good damage/leadership ratio + 2 Support units (like Archmage + Paladin or Archmage + Demologist or Archmage + Inquisitor).
This is why my ultimate party is:
1 Tank: Knights
2 Damage dealers: Paladins and Horsemen
2 Support units: Archmagi and Demonologists
This party guarantees no-loss victories for me all the time. Units are very high on HP, most of them have decent multi-type resistances. Good vs. Practically Everything.
If you feel bad with this party, how about this:
1 Tank: Paladins
2 Damage Dealers: Lake Fairies, Tree Fairies
2 Support units: Archmagi, Dryads
Low on HP, nothing much in terms of resistances, but the damage output is great, as long as you're not facing dragons