focus only increases magic damage like spells with X-X Damage of Y Element.
it doesnt increase anything with weapon dps.
That is wrong.
Focus and STR both increases %Weapon DPS by 0.5% per point for their specific damage type (Physical for STR, and magical/Elemental for Focus).
If an ability does %Weapon DPS as physical, then it wont be increased by focus, if it does elemental/magical damage however, then it IS increased by focus. Same goes for What type of damage your weapon has.