For this to work to the optimum one player
should have a sword. On to the tip: we shall call the player with the sword Player Two(P2) so first of all get P1 to go up to the prophet and board him now that P1 has broken the prophets shield get P2 to lock on to the prophet with his sword and lunge at him it will do much more damage then a melee atack and kill him quicker. I once told my friend this and he didn't believe me because the prophet has shields but you must keep in mind that P1 has broken through the shields it is best to do this with a sword because a weapon as powerful as the sword would be a rocket launcher but that would kill P1. There is another way which is get P1 to penetrate the shields then get P2 to throw a plasma grenade on the prophet and then get P1 to get off the prophet. Once P1 is off the prophet the grenade explosion is actually stuck in the prophets shield also if you really want to be a daredevil do the plasma grenade steps but throw a frag instead if P1 gets off at right time the shield will come up as the frag bounces off the prophet and frag will be stuck in the shield. I have tried and tested all these methods the frag one is very hard but can be done another good thing about this technique is that both players are involved in killing the prophet. This technique is very good for higher diffuculties like heroic and legendary.
I hope this helps some players trying to beat the prophet in legendary.