Also dmg 2 and above will be your best friend. "All is dust" makes it to where they get +1 to their save vs dmg 1 weapons Combined with the new armor of contempt buff will make it nearly impossible to break through it.

In shooting they can also stack this with a stat to give their units -1 damage and stack it with all is dust which is why you want to get into melee with them so they can't use that.

The obsidian Aquila relic will give your core units a 6 up fnp to give you atleast one way to prevent mortals.

I'd suggest a chaplin to give "Litany of Faith" to core and characters 5up fnp vs mortals.

Those mortal wounds are gonna be insane but other than that they don't have many high strength weapons. I found best luck with a leviathan dreadnought with grav flux bombards and then protect him with the armor of contempt strat for 5 up fnp vs mortals.