"They will either summon minions every single turn behind your defenses to insta gib your ranged units,"
Well this is why in master of magic, there was a summonning circle, that could potentially solve the problem.
"or they will simply use AOE damage spells, that will insta kill your weak ranged units even if you put the elemental resist spell on them."
The problem is that elemental resistance spells are not powerful enough. MOst unit enchantement are pretty weak to pay 1 mana per turn to maintain it. I though that win walking allows unit to move on any terrain even water, but it only remove terrain movement penalty. I rahter pay 2 maintenance to have a unit of imp on the board than having 2 units get +25% elemental resistance.
Most 4x games seems to work like that. Still, they sometimes add a corruption or unrest element to the game so that the player has other things to manage. Unrest could possibly be a good idea. In MOM, there was unrest, but it was city related. I like a game where smaller kingdoms till have a chance to win due to strong magical power or political power. But I don't think it will happen for now in warlock.