For those of you who don't know Bulltrue is a medal for killing an enemy who is in mid launch with either the Energy Sword or the Gravity Hammer. Obviously the Energy Sword is easier to find, so if your boosting this achievement, which I recommend doing there is a very simple way to get this achievement.
To lunge with the sword or hammer press RT when your reticule is red. (not B as B is to melee with the sword).
Maps which have energy swords, HERETIC and LONGSHORE
Have 1 person on the opposite team get the sword and get whoever is getting the Bulltrue medal to stand in front of him with an Assault Rifle. Have the person with the sword move forward until the reticule turns red. Then have the person with the Assault Rifle continuously fire at the person with the sword. Once the person with the sword's shields deplete have him lunge at the other person. Keep firing the Assault Rifle and you should kill him just before he kills you.