If you are holding objectives, sniper scouts might be a good idea.
Since you are camping, definitely load up on long range death:
Typhoon speeders, Thunderfire cannons and of course Stormtalons are an option.
If you are not dead set on the Chapter master, Tigurius would be way better to buff your defence.
Since the Grey knights will move upfield, Grav centurions can also move up behind them and camp midfield.
On the above list, i would seriously consider naked sternguard veterans instead of the assault squad.
I do not think there is a unit that can defend better a bolstered ruin than stermies, since they can bypass 4+ armor from 30" away