I'm going to share with everyone "Tenebrium transformation" trick I figured out.
First, convert regular weapon to Tenebrium one, by adding Tenebrium bar. Tenebrium damage boost is greater then boost from adding elemental essence to the weapon.
On the other hand, if your character is dedicated to regular weapon skills, this may not be such great idea, since you will lose on total damage dealt, due to any weapon with Tenebrium using Tenebrium weapon skill instead.
Now comes the trick. Use elemental essence on this kind of weapon to add some kind of elemental damage. Result will be that elemental damage boost will add same amount of damage as Tenebrium did.
To sum up: (Weapon + Tenebrium Bar) + Elemental Essence -> more elemental damage then Weapon + Elemental Essence
EDIT: One important thing to add. Weapon skill increases just base damage of the weapon: Slashing, Crushing or Piercing. It does not affect any additional elemental, poison or Tenebrium damage that weapon can have. This is true for both regular weapon skills and Tenebrium weapon skill. In case of staffs, base elemental damage of the staff is also modified by the skill.