Ask Tingalz how easy that is to accomplish
Kill the pathfinders first.
Bring lots of anti-tank and ap2 to use against his hammerheads and broadsides.
Have some good anti-infantry such as heavy bolters to use against his fire warrior blobs.
Do not put all of your vehicles in the same spot. Better yet, don't bring any vehicles and laugh as his railguns become near useless.
And for forgets sake, try to get the initiative.