There is no problem using any rocket launcher when hiding under the slope. It take care but is easily doable. Either go to the end where the two boxes are offset where you can jump out and jump-and-fire at Valus, or the more tricky hide under the slope right by the first box, crouch and aim at Valus and fire (making sure no mobs run in front of you....). Both methods are better with tracking rocket launchered for obvious reasons but it works with any launcher. Not cheesing, it's tactics. Bungie put the boxes there deliberately and this kind of thing is a trademark of their maps. Halo games were littered with just this kind of thing.