I typically protect my Orca and backfield with my heavy infantry.
Just like he said above protecting your vehicles with a layer of troops is very useful.
It might hinder movement a bit, but I usually get my heavies into position to cover everything. I don't take light infantry as they just don't hit hard enough nor are they particularly good at CC. Heavies are both. Those chainripper bayonets make cleaning off melee units from the Orca's heels less of a hassle. So does the Falloway slam.
On rare occasions when my center line is solid (and not in chaos), I will send my orca out on a flank or on its own seek and destroy mission.
I seem to always pick the multi rocket launcher for TTB:1 to increase the RoF. As in most occasions it has the best area coverage. But this is highly situational, though the pull is there every time I activate the dude.
Against a melee centric army I would use the orca as my center and screen him with my heavies. If the screen gets broken then they still have another round of shooting to deal with from the orca.
Against a ranged army there's not much to be done but try to find some cover and keep them at the edge of the 30inch range band for as long as you can. The orca is not a vehicle killer. He seems to me to be very anti infantry.
Against heavy rd army the orca is not a very good option, but can weather an attack nicely if there is minimal avv. If not then kiss it goodbye.
So to recap. It's a troop killer used to clear massed troops, take potshots at solos with the hmg, and solo/squad leader hunt with the crit force 2 auto cannon. Keep him away from melee or screen him well. Especially weak against crucifiers...
