This can be a difficult achievement to obtain simply because a lot of hits that take you to less than 5% health are going to also hit you hard enough to simply kill you.
The best way to go for this achievement is in Battlegrounds. You will have a higher chance of shear battle in this mode, especially in team death match.
I recommend using a lot of AOE skills when there are large groups fighting, which will happen in Battlegrounds. Just go ahead and lay down all your AOEs and then get in the fight.
With some luck you will eventually experience a moment where you are below 5% health and one of your AOEs may kill someone during this time. Again, Battlegrounds is the way to go.
Another method is to have a friend in a different alliance. Join an xbox live party and enter the same multiplayer campaign. You can communicate to meet somewhere on the map with little activity from other players and simply get one player to under 5% and let that player get the kill. Exchange turns to both get the achievement.
Hope this helps! Enjoy