Jackals' Unlimited Plasma Pistols
Play the Sacred Icon level until the player reaches the point where their Jackal and Grunt allies are fighting Sentinels. Destroy all the ports that the Sentinels come out from so they are safe. The player should grab a Covenant Carbine if they haven't already got one already, and trade it to a Jackal (make sure the other weapon is not a Plasma Pistol or the player won't be able to trade with him).
Then, they must charge up the Type-25 Pistol and launch it at the Jackal's shield. If this is done the first time, the Jackal will freeze. This will bring the shield down temporarily and he will drop the Type-51 Carbine and pull out another Plasma Pistol. You can do this over and over and the Jackal will pull out a new pistol every time, thus giving you unlimited Plasma Pistols.