No, there is no way for the AI in any halo game to assasinate his foe. The characters are not scripted in halo reach to activate their "animation" and assasinate you. As for other titles, thats questionable and would require testing. I don't recall any enemies use melee is other titles except a couple of brutes with their gravity hammers, and the elites with their energy swords. So to the best of my knowledge no, the enemies can not assasinate you.