I had posted a bunch of similar stuff in another thread, but that one quickly went off the rails, so I'mma copy-paste is all here.
-I've heard stories that Wolvers used to have better tracking when attacking, bringing that back would be a good start. Chromalisks should spawn cloaked, cloak more readily, and maybe have a slightly longer attack range (but still attack from close-up).
-Slimes could have their elemental identities strengthened. Self-igniting Oilers, Toxigels that spew poison gas (or leave poison blobs like in Compound 24), that kinda thing. Low-level Lichen colonies could stand to be more interesting, too. Maybe if they got a speed boost the smaller they are.
-Retrodes rays could explode a little faster, and Skuttlebot attack area could stand to be larger (and maybe be a true cone, rather than a bunch of bullets). Although I do think that enemy Mecha Knights need a longer wind-up time on their generic sword swipes, but the random self-shocking can go away. (And if I never see a Rocket Puppy ever again, I wouldn't shed a single tear.)
-Gremlins are mostly fine, although the little ones could stand some kind of buff and the Flamethrowers' flame trails should chase players rather than being random. (Same for Flame Puppies.)
-Mewkats are fine as-is, but Zombies should get the same knight-tracking buff as Wolvers for their leap and breath attack. As for Phantoms, see my previous comment on Mecha Knight attack speed.
-Fiends need no help. Devilites should be nerfed if you ask me, but that's another topic.
Or you could have them focus on what they are GOOD at and be balanced,not virtually the same and balanced, just saying balanced means different but equel not same and equel. Example, Beasts are power mongers, and jellys are defense tanks.