Shadows: Awakening

For this you need to level a skill to 3. It costs 6 skill points total, and cannot be unlocked until level 10. This unlocks the 3rd level of the first skill for each character. You can use the shadow traders to respec for a small amount of gold, so you can get this with the first skill and then refund the points if you don't intend to use the skill, and want the achievement ASAP. However you get plenty of skill points and the fact you only get 3 slots means you will probably find 3 or 4 skills you love and just pour your points into that, and you'll have a bunch left over.