As the achievement description states, you must win a duel without taking any damage. This is an extremely easy achievement to boost with a friend or by using a second controller as it only takes a few seconds to complete a duel. If you are boosting with a friend, ask them not to attack you back. If this is the first duel you have ever won then you will also receive the Duelicious achievement.
To start a duel with another player simply join a co-op game and then walk up to another player and melee them to propose a duel. They will need to melee you back to accept the duel.
If you have two controllers then setup a local game in the same fashion and duel your second player.
If you don't have a friend or a second controller to boost this with, try to learn the strengths and weaknesses of each class as you play co-op and use that to your advantage. You can also try to start a duel with another player that is a few levels below you and that will give you a strong advantage.