Well that is a problem... unless you're and orc... with several million other orcs... then you can just rush them, its just a matter of dividing the loot after. Imperial guard could do that if their morale held up to the massive losses.
Well considering the fact that asteroids several kilometers in diameter would break up and still make it planetside would sort of make my earlier scenarios useless... especially since it would kill everything in 1000km radius. Regardless i suppose the orcs would just keep pouring wave after wave into the Tau regardless until they all died or all the Tau died. I'm ignoring tactics here since orc aren't very intelligent to begin with.