One of the ways to make humans more appealing than mythos is to have monsters demand tribute every couple of turns. The way I thought this could be done is using a hunger mechanic. Mythos would have a hunger meter, and every X turns (could vary by mytho) they would need to get to a town and raid it for food (or some other resource). This would make the placement essential: mythos need to be placed because they can't stay and fight for too long.
Additionally, the hunger meter could affect the mytho. Once a mytho is 'hungry', they could get angry. A giant could flatten a mountain out of anger, or even eat another unit. But, they would also get weaker as their hunger went up. At some point, they'd be so weak that one or two regular units could beat them. This would mean that you could deliberately starve a mytho (by putting mountains/swamps in its way for example) in order to take it out.
