Ork firepower is virtually useless, shooitng at marines with orks IS useless. Assault them, but do not give them a chance to shoot you, so get there fast, how you do that? Bikes, transports, infiltrating and deep striking.
Bolter fire will decimate boyz on foot, lascannons and missile launchers will crush expensive dreadnougths and war machines, you have to get boyz into assault and keep them there.
A looted leman russ would not a miss, or 3.