When a "swords" match pops up, wait a second or two before rushing into the fray near the center of the map. One or two people will be stunned from a parry (two people simultaneously lunging), and you should be able to kill both of them before they can recover to fight back. Optimally, melee one in the back and lunge at the second (The melee takes slightly less time, so it's less likely that you'll be countered.) After that, you have four seconds to get a kill, so in a 6 player match on a small map, that should only take a small bit of luck.
I got it on Epitaph, but it can be done on any map.