With reference to the Wiki section Ralath pointed out previously, here are some additional information for you:

• As for license progression, the monster must not be too weak for you; it must also be less than 20 levels above yours (Golden Bat is a typical example: it is weak enough to grind on for Lv 5x, but it is indeed Lv 76, and if you're not Lv 56 yet you won't make any progress on its license)

• This bonus% applies to the base damage on the weapon. In other words, if you're wielding a +9 weapon with a large amount of +damage, this bonus may not be as significant as you think. When applied to the monster, the actual damage bonus is certaily much lower than the % advertised on the item's hover-menu

• Check out the maximum crit% bonus and number of kills required for a license in the Wiki. Make a note of the monster level too :)