Instructions
Equip a shotgun, wait for the sword-wielding enemy to begin his lunge, then fire. A shotgun is a one-hit kill at close range and is quicker than the sword.
Throw a plasma grenade at the sword-wielding enemy as he approaches you. If the grenade makes contact, it will stick to the enemy and guarantee you a kill.
Equip a rocket launcher and fire at the feet of the lunging enemy. The splash damage caused by the explosion will kill the enemy and award you a "Bulltrue." Be forewarned, this method will often result in both you and the attacker being killed.