I want to create a spell that makes targets more vulnerable to damage of all kind for a few seconds.

Simply adding a perk that modifies incoming damage by, say, 1.25, would be great, but seems it's not possible to add perks to NPCs through magic effects. I tried both "Perk to Apply" option and "target.addPerk(xxx)" in a script.

So, the other alternative is to create a script that temporarily decreases the folowing statistics by, say, 25 points:

• DamageResist
  
• FireResist
  
• ElectricResist
  
• FrostResist
  
• NormalWeaponsResist

I know decreasing the elemental resistances will really make the target vulnerable to elemental damage, but I'm not sure what exactly decreasing DamageResist and NormalWeaponsResist will do.

So, what you guys suggest to make targets vulnerable to all damage? Is there really no way to simply add a perk? And what about the resistances, can that work?

IIRC, you cannot ever add perks to NPCs. Player only.