Enemies scaling off of the stats you have is not a good idea, as it makes stat progression meaningless.
I'd instead propose a "world level" in which enemies' base level increases based on what your current world level is, and maybe you could spend Juice to increase it, which would then increase your progress rate. This should also effect bosses as well.
I also like the idea of using "Core Stats" to improve them (Such as Muscle or Brains), while you could still invest Juice into advancing any of your "Minor Attributes" such as (Health, Health Regen, or Melee Damage).