I would check if there’s a passive template that allows flat absorption, then you can make a 200 level passive and give it to enemies with level of skill == char level condition (this way various pets scale their armor e. g.). The downside is flat absorption works against physical damage as well so you’d have to review armor… Also, it’s interaction with DoT damage is pretty silly, so it necessiates extra tuning.
The somewhat roundabout way is to just flatten the damage scaling. If your skill is doing 10 damage at level 1 and 1000 damage at level 100 because there’s 80% resistance then you can just rescale damage to 10 at level 1 and 200 at level 100, and removed all sources of resistance.
I’m puzzled why you want to achieve something like that. Part of ARPG’s fun is the feel of scaling up.