So are covenant troop ships. We are thinking of humanity and covies as one army. The shields is an interesting point, but both human and alien forces have used EMPs alongside some of their salvos. Also, ODSTs are deployed in the hundreds for assaults and are trained to be guerrilla fighters. A single soldier with a grenade, cutter and grappling hook had no problem taking down an AT-AT and neither should an ODST with satchel charges energy blades and magnetic gear. Your also forgetting the SPARTANs and normal Marines to add as force multipliers.