this is by far the easiest way to level but it is also a bug exploit and kinda slow: complete the skellige succession sidequests (given to you after you get there, and do a tiny bit of the main quest), then you get invited to a party, and there is fist fighting there (to clarify that is the second party with fist fighting there... yeah)
the male contender for the throne will challenge you to take on a fighter (you have to talk to him or the fighters, i don't remember), do so and win (you get a sword), after that you can speak to the fighters again and challenge them yourself this time, if you lose the fight you get to try again, however you get 50 xp even if you lose, so you can repeat indefinitely (i noticed this on the unpatched gog version, but i doubt the patches fix it, haven't looked through the notes though)
also monsters less than 10 levels above you shouldn't be a problem because the combat is ridiculously unbalanced in your favor (tip: add a bleed effect to your sword, it scales with enemy hp, and thus does not suffer from you being underleveled)