A skill's profficiency is upgraded by using them during your travels. The easiest way to level a skill to its max is by using the Healing Bullet skill and go to a dungeon boss. Make sure you're not the one getting hit and let your friends do the rest. They should get fairly damaged and you will be able to use that skill a couple of times!
Use recharge speed equipment and items and make sure to have your INT stat fairly up to use a skill often.
This achievement takes time if you're solely focusing on getting it. I achieved it by just playing the game without farming. Good luck!