You need to protect your hero. If he dies, you can consider the game lost, as the hero's tactics are crucial for supporting the army. Monks counter fairies. Squires, spearmen, and champions will fight phoenixes, manticores, griffins, fairies, and summoned creatures. The Crusader is indispensable here, with his excellent double strike and bonus to hero attack and defense.

Haven only has the Djinn as a flying unit (though they mostly use spells).
Necropolis has the Imp, Ghost, Vampire, and Devil (or Bone Dragon).
Life has only the Angel.
Nature has fairies, griffins, phoenixes (or magic dragons, but they mostly use spells), and manticores (or air elementals).
Fortress has harpies and storm birds.
Chaos has ifrits and black dragons.

Conclusion: Monks are good against Life and/or Haven. You can use either Monks or Crusaders against Fortress and Chaos. Against Nature and Necropolis, only Crusaders are effective.