Maybe someone already offered, but I also offer this idea.
Monster troops-1big giant unit who offcorse more endurand then other troops, have a few health points.
I have some offer what monster for what fraction:
Orcs-Ogre or Troll(I don't know what size they have)
Dwarfs-War Golem
Elfs-Ent(Better no to think)
Humans-Elemental(Don't know what element but I think better fire)
Undead-Or big Skeleton giant or Flesh golem
Legion-I don't know what pet they need, but how about Cerberus