I've played against horde Orks before. I've found that taking out those Lootas should be top priority. Else, just snipe out any Weirdboyz he might have, truckks, and any other mobile unit, and you'll find you can ignore the massive hordes of Boyz quite easily. Beware Weirdboyz. I've had a game where one jumped into my backfield, then cast a power that forced my warlord to take a Toughness test or be removed from play. That cost me the win.
"Death solves all problems. No man, no problem."