From Halopedia, the Halo wiki
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Bulltrue is a medal in Halo 3, Halo: Reach, Halo 4 and Halo 5: Guardians multiplayer that is awarded when a player kills another player who is in the act of an Energy Sword lunge. The lunge does not necessarily have to be directed at the player who receives the medal; the swordsman can be in the act of lunging at any player. In Halo 3 and Reach it is represented by a brown diamond with a silver outline and a bull's head in the center. In Halo 4 the bull's head is atop a blue circle and grey cross. In Halo 5: Guardians, the bull's head is on top of a brown octagon.
The medal accompanied with the message, "Bulltrue!".
It is actually a very challenging medal to attain, unless playing Grifball or against inexperienced players. This medal is slightly easier to obtain as a Juggernaut, as increased shielding and damage resistance lead to a non-one-hit-kill with the sword. A popular yet difficult way to obtain this medal is to hit the sword user with a sniper headshot at close range. Another way to receive this medal is to play Infection or Flood.