in town go to merchant ( Demetra ) she can upgrade your skill to certain point for very low cost ... easies way to get skill to lvl 20-30
later become more expensive ... but if you want to try new skill lvl1 this is only fast way

Thank you, yes after certain level around 50-60 just impossible to pay the price :(

High level mobs will level up a level 1 skill very quickly at first. If the modifier you want to test is not a high level one then it will not take long at all before you unlock it, save your essence in that case. If the modifier is a higher level one, like level 41, you are better off saving your essence until the skill has been leveled to about 37 or 38, then spending some essence to gain the last few levels that unlock the modifier you want to test.

To give a simple anwser to the original question:

Skills are XP bound, more XP from mobs = faster skill leveling. And no, you don't even have to use the skill, as long as it's part of the skills you selected on your skill bar, it will level up regardless of if you use it or not!