One way is if you're lucky enough to get an objective gametype on a map with the sword (e.g. Oddball, King of the Hill), ignore the objective, go straight for the sword and wait for three bleeps on the radar.
Personally, I got it on Construct on the top floor near the purple grav lifts. You'll eventually get it. Just wait for one to come up the lift, kill, then move to the other lift and kill any enemy in your path.