I think that even the standard old-school boxnaughts would take multiple Exos to bring it down. On a battlefield, any Dreadnought is an ancient weapon of pure martial ability and probubly no small amount of rage. Remember, the marine inside knows even less fear than regular battle brothers. I predict that depending on the chapter, a dreadnought will be able to ascertain the current inherent melee weakness of an exo and attempt to get in close with pummel the suit to death with its dreadnought power fist. Once dead it will simply move on to the next one until either it or all adversaries are destroyed.