To elaborate further I've been thinking about distinguishing the enemies with
a dark colour and a red outline.

I might use certain designs for harder to dodge enemies. Currently you can outrun them all but I might make some closer to the players speed.

e.g. the Slime would move slow, the mouse like here here would move faster.

Currently I have it so enemies stop pursuing the player if the players jumps up to or falls down to a lower platform. I might put a flying enemy type in as well that ignores the terrain and can trigger a battle while the player is in mid-air.