A level 20 skill is one at the end of the skill lines for a character. To unlock it you'll need to fulfill a few conditions:
- Get a character to Level 20 (Librarian doesn't count)
- Put at least one point in each skill/trait leading up to the one at the end
- Put one point in the Level 20 skill
In general, this one won't take long if you're going to complete the whole story mode and especially the rest of the achievements. To speed things up, consider doing the following:
- Turn up the Experience Bonus to 300% and the Loss Penalty down to 0%. This will ensure you level up much faster.
Slak will likely be your first character to hit Level 20. Just make sure to put a point in each skill as you level up so the skill will be available when you hit Level 20. If you don't, just respec you character and assign the points to unlock it.