This is the second to last skill in the swordsman path. It unlocks group finishers, and it requires at least eight talent points to unlock. First off, you'll need this skill for another achievement ("Threesome"), and secondly, I would strongly recommend keeping this ability, as it's a lifesaver. However, if you don't want it then make a save before spending the talent points and then reload the save once the achievement unlocks.
The achievement will unlock upon exiting the upgrade screen after purchasing the skill.
You can get the "Swordmaster", "Mutant", "Master Alchemist", and "Master of Magic" achievements in one go, by using the From a Bygone Era side mission on Iorveth's path in the correct manner. Follow the walkthrough here.