There should be burrowing monsters, that can pop-up behind your front lines. There should be hidden monsters that ambush your units as you pass their tile (and then they become revealed). There should be monsters that can become hidden if they move into their favorite terrain and none of your units are near. (aka hidden bug monster can flee to a piece of forest far away from you units after ambushing, allowing it to become hidden again, and desert monster can flee to a far away patch of desert)
There should be spells that can reveal hidden monsters, and spells that can block the burrowing of burrow monsters. (aka Wind of Revelation vs hidden monsters, and Under-Earth Stone to block burrowers.
There should be ONE powerful burrowing monster that can ignore Under-Earth stone, and there should be ONE near-invisible monster that can ignore wind of revelation. These monsters should have counter-spells at a much higher level (like a level 6 wind or earth spell).
Most ice monsters are weak to fire spells (and immune to water or ice spells), however certain powerful Ice monsters can ignore fire spells that are level 3 or weaker, neutral to fire spells level 4 and 5, and weak to fire spells stronger than that.
Most fire monsters are weak to ice spells (and immune to fire), however certain powerful fire monsters can ignore ice spells that are level 3 or weaker, neutral to ice spells level 4 and 5, and weak to fire spells stronger than that.
I think relation of Wind monsters and Earth monsters should be similar. (however lightning spells are always worthless vs earth monsters, and always good vs water/ice monsters?)
Most flying monsters should be weak to wind attacks, however stronger flying monsters should be immune to weak wind attacks. (A flying Dragon would be immune to Wind spells level 1-6, neutral to wind spells 7-8, and weak to wind spells level 9 or higher)
Most burrowing monsters should be weak to earth attacks, however stronger burrowing monsters should be immune to weak earth attacks. (A burrowing Dragon would be immune to earth spells lv 1-6, neutral to lv 7-8, and weak to earth spells lv 9 or higher)
This relationship between flying monsters and wind attacks should only be for wind attacks whose area of attack includes space above the surface of the ground, and relationship between burrowing monsters and earth attacks should only include earth spells that effect anything below the surface of the ground.
For instance, an earth spell "rock fall" wouldn't effect a monster underneath the ground, however Earthquake would cause double damage against a monster underneath the ground.
