Here's what I would like to do. I want to make actors completely ignore weapon's damage value, but ONLY the weapon's damage value. I still want enchantments to take effect. I still want spells to take effect. I still want falls and fire to damage the actor. I still want the actor to be mortal.

I also want to be able to apply damaging spells and effects via script on that actor.

How can I do that? Thank you!

I haven't really tried it yet, but I know it's possible.
You can try to use perk, and add it to said actor. With perk system, you can change even hard-coded functions

Or, you can look at the script of the mages Guild - Ancano during the Eye of Magnus quest, which he was invulnerable to all type of damage, except some spell from the staff of magnus

Ah, thank you

From what I can tell, the quest script sets Ancano to be invulnerable. I don't even know where it allows exceptions to damage him at all.

Ok, sorry for sending you running around

The one that really helps you is the Perk. I couldn't recall what perk was it, so I have to fire-up the CK and look it up...
Remember the perk entry for 100 light armor? Make you invulnerable to melee damage ?
The entry for such perk is

'Mod Incoming Damage' == 0.00

Ah, now that is very handy! Thank you so much!

Hmm, I wonder if there's a way to activate that through a script? ;-P

No you can't active that via script, sorry
But you can add function to such perk, like whenever to active that perk (quest stage, spell effect, etc...), or to whatever type of enemies that actor is engaging in combat, etc..... All can be done in the condition function window of that perk.

Haha, they would make it so it weren't so easy to do as a script, eh?

That's ok, thank you again!

Now, I just need to figure out how to get every actor to have that perk successfully and without TOO much work

Sounds like I need SkyProc again!!!

Good luck, just a head up: You will run into trouble with AddPerk that doesn't work on NPC, or any Actor, save only for Player.
You can search a thread that Spookyfx ran into it. I also suggested a work around in that case that 'might' work.

Or, you can always use 3rd party hooks like ScriptDragon or SKSE (in the future)

Sounds good, thank you!
Yeah, I can't believe the AddPerk command is broken for NPC's!

I am prolly going to have to use SkyProc anyway in order to get my script attached to all actors. So I hope adding perks the same way will be possible

Perk entry points don't apply to NPC only to the player, but you can always use perk entry abilities(see dual flurry)

huh?, what do you mean Perk Entry Points don't apply to NPC? Did I misunderstand your meaning, or you really mean that NPC cannot use perk in game?

They can't use the predefinied entries, like "mod weapon damage value=http://forums.bethsoft.com/topic/1355825-make-actors-ignore-weapon-damage/value*0.5"

Hmm, I'm not quite sire of the technicalities of NPC's and Perk Entry Points, but I duplicated the Deft Movement perk, edited it to have no conditions, then applied it to a few NPC's. The result is that normal weapon damage does NOT harm the NPC. Bleeding effects and knockdowns still apply, though. So I'm a happy camper now

Daedric armor is now something to be feared