Thanks - the advice about Faith Points and forcing the player to burn them is good. I will remember that.
I never felt at risk from his army, just that the mission (combined with my list) made it almost impossible for me to score a win. Did I have enough firepower to table 60+ power armoured models? Not unless I was very lucky with my rolls, and he unlucky with his. Considering the +1 cover and hiding out of line of sight and I would need to get right up in his grill to wipe him out. He would always have a couple of squads left (the other six plus the Seraphim would serve to delay me so I could never close on his home objectives and rapid fire them) to claim two objectives, making a draw almost the best result I could hope for.
So, I decided to have a little fun, and wound up with the draw anyway