Most obnoxious regular enemy award goes to:
Primal Aspid
Though, only because there is no upgraded version of these bastards:
Crystal Hunter
The triple-goo attack from Aspids, with the center ball always targeted directly at you, makes it impossible not to get hit if you go for an attack while they're shooting. At the same time, with their high firing frequency and tendency to keep you on distance it's hard to find some timing.
At least they die in 1-hit to Shade Soul with Shaman Stone charm or Charge attacks (with maxed Nail).
Meanwhile, Crystal Hunters are just pure bullshit in platforming areas already filled with spikes. That path to the Descending Dark upgrade in Crystal Peak is just the worst. On my first playthrough, I immediately attempted this after getting the Crystal Heart and, boy, did it cost me a bunch of lives.
I love how you can actually reflect their projectile...only for that to barely matter because they don't get hit by it and you still can't jump into the crystals it creates.