Have it so not only slimes spawn gems but mid tier enemies spawn higher ranking gems to spawn high tier enemies. Makes it feel like less of a waste when a unit dies, and encourges you to use more units.
Once you add more the the game you can have vampires increase power from killing mobs, so you will have to think about what your spawning. However make it so each vampire has a enemy they are weak/strong against. So mass spaming units does not become the best option. As if you mass spawn the units and they die they might level up a vampire. However make sure to not overtune it and turn the game into a puzzle game. Maybe even have a "level" cap that increases as game time does. So early on even if your units spam die to a vampire it won't make the vampire unreasonably powerful.