This is for putting 15 points into one skill, requiring you to reach level 16 at least. In my opinion, it's best to go for this on a rogue or mage character. Both classes have one skill that's far better than the rest.
On a Rogue, put one point in stunning strike, but no more than one, because you want its mp cost to be low. Put the rest of the points into poison, increasing your damage. You'll get the achievement after the character reaches level 17.
On a Mage, put one point into each spell, and the rest into Stream. This increases the damage without increasing the mp cost. You'll get the achievement after the character reaches level 19.
If you bought the Miniatures ( Permanent effect : +1 to all skills) then you'll need to go up to 16 points in one skill, not 15.