This achievement is much more difficult than it sounds, but you can get it easily by abusing checkpoints. Use Marie to pick off archers (usually a one hit kill) whenever you can while playing through the game, but don't worry to much about it until you get very late in the game.
When you get to the chapter titled "Montsegur Castle", which is Chapter 19 (second to last chapter) you can easily get this achievement.
A short way into the chapter you will have a fight in a huge open room. After the fight, follow the gold arrow down a hallway to the left. You'll see a door to left and right. Soldiers will ambush you from the door on the right, so kill them. Go in the door on the left (a library) and open a secret bookcase on the wall. Go through into the treasury. Open all the chests in here. One of them will have the Free Dagger Throw Boon for Marie.
Exit the treasure room into a trap room. Make your way through and follow along to a large hall with a fountain. In this room there will be many Templar and Inquisition forces fighting each other. Switch to Marie and throw daggers like crazy. If you don't have the achievement by the time everyone is dead, just reload the checkpoint and you'll be right back at the beginning of this room. Throw daggers like crazy.
Rinse and repeat until it pops.