For this you must get 5 sword kills IN A ROW IN ONE LIFE. don't be tempted to use stickys or anything. This can be a tricky achievement with a shotgun on the map and ALOT of plasma grenades, the key is be patient, act like a ninja. wait around corners and don't hesitate to hide and be cheap, if you charge someone they can easily run backwards and shoot or stick you, just be patient. If you lose sword, don't get frustrated, just grab the shotgun and kill him or ask for your team-mates help.