As it says you must defeat 100 opponents without dying.
Spend the majority of your time blocking.
Always try and counter kill them as it is a lot easier.
Run around the city killing every guard you see.
If you are having trouble, this glitch works for some reason:
At the start of the game in Masyaf, run along the far left side of the village until you see a spiked outcrop pointing up toward the mountain with a tree just in front. If you run into the corner at the point while moving left and right, you'll be able to wriggle up the cliff face. When you reach the top, jump off the edge of the map and you'll unlock the achievement!
Credit goes to Mullock94 for the method and Moonska for the video.