Do you have to use a close combat weapon for the spell failure ability to work or can you use a ranged weapon and get the same result?

You can use any weapon. I've disrupted mages in the middle of spellcasting with just a basic longbow and normal arrows before. Insect Plague is a good spell for disrupting also.

I didn't rate the wizard slayer as he can't use healing potions etc. very awkward.

What i ment was the special ability the wizard slayer has, 10% cumulative spell failure for each hit.
What if i hit the wizard with 2 arrows? Do he get 20% spell failure or not?
What if he has stone skin? I hit the wizard but do no damage, do he still get 10% cumulative spell failure?