how to make certain enemies incorporeal to all weapons except a few categories and spells, like ghosts in Oblivion?

Hmm... That's a tough one.

You can easily stick a perk on them using a Mod Incoming Damage Multiply 0.00 entry point with contionals for HasKeyword WeapTypeSilver OR and HasKeyword WeapTypeDaedric OR run on the Attacker Weapon.

But that wouldn't include enchanted weapons. There is no way to single-out a weapon's enchantment and work with it. However, in attempting to do so, I did find a way to detect if a weapon was enchanted. If a target has a weapon equipped that is enchanted, then conditionals run to see if a Contact-ranged spell is equipped will return true when run on the hand that enchanted weapon is in. This means when an enchanted weapon is equipped, you are considered to both be using a weapon and simultaneously be using a contact-ranged spell. Use this method to detect enchanted weapons on that perk, and you'll officially have an Oblivion-style ghost that's immune to any weapons that aren't silver, daedric, or enchanted.

thanks, but what I mean is also the effect of weapons cutting through them like if it was thin air
maybe that in oblivion was done due to the "resist normal weapons" magical effects that is absent in skyrim?

Setting the Ghost flag on an NPC to true causes all weapons and spells to fly right through them without trigging a hit. There isn't a good way to turn that on and off for each attacker, though. The best you could do is set the ghostly NPC to not have any blood spraying from it, then use the above method to make it immune to weapons that don't fit the type.

As you said, Resist Normal Weapons no longer seems to be a thing.

unless they add it via other DLC
then if the ghost flag is set on a npc how can one hit it?

You can't. That's what it does.

so it's only for "decoration" ghosts thanks!
the "resist normal weapons" script you described would not work in the exact same way, right?

That would not be a script. Just a perk.

And it would work in the same way that Resist Normal Weapons worked in Oblivion. Just make sure that the NPCs you stick it on are set to not bleed, because that would be weird for a ghost.

what about making it a magic effect?