It's quite simple, you just need to understand skills go up the more you use them, same applies to attack abilities.

It's important to distinguish abilities though, since most can't pass walls and some can. This method applies to those which cannot pass walls.

The method is simple, aggro an enemy near a wall, and let it sit on the other side of the wall. Now spam all your magic spells nonstop, since you can't hit the enemy you can keep on casting and leveling your skills. This doesn't work with earth and darkness since those can go through walls and kill the enemy, and attack spells can only be cast if there's an enemy nearby.

Hope you find this mini guide useful, it can save you a lot of time if you do it correctly. For example check the screenshot that shows all the spells being used in succession, making real easy to level up fire, ice, light, magic regeneration and magic destruction.

Bonus trick
For the melee skill, it doesn't level up if you attack empty space, you actually need to hit something. For this I'd recommend equipping your lower damage dual wield weapons and go to any area with high level monsters and keep attacking them. It should take a long time to kill them, but everytime you hit them your skill should go up a little, keep at it and it should rise quite fast.