To earn the Swordmaster Achievement you need to unlock the Combat Acumen ability. It’s on the Swordmanship Skill Tree. You need 7 talent points to unlock it. Once you do, you’ll get the achievement.
Note: If you don't want to invest in this skill, make sure you save before you spend the talent points. Then you can go back to your save with the talent points intact.
Note: You cannot earn this while playing the arena mode.
Note: 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.