If he summons 20 ghouls and a varghulf, he couldn't have had the extra attack spell on him, as he summons on the end of his movement.
But yes, if he managed to put that on 20 ghouls, with feeding frenzy and a king wholly within 18" he'd be able to reroll 1's to hit. Basically, you'd have to be wary of pile-in ranges, you know FEC by now. Just place your juicy targets further behind your clanrats so he can't possibly pile-in and be able to hit, so you get an free attack with your juicy stuff behind if he does.
Another tip, shoot his leaders. Death armies in general fall apart without leaders, he'd lose the 6++ save, the ability to feeding frenzy, etc. And with MMWP you should be able to kill the courtiers from a distance at relative ease.
MMWP/Vigordust melee stormfiends should be able to kill plenty ghouls, as they have a save of 6+ and 6++. Also, a terrorgheist falls fairly easy to 40 plague monks. All in all, I don't know how I'd fare (quick math, you'd kill 17 ghouls on average with buffed clubbing blows, that's without the 2 actual melee stormfiends, considering you play them as a unit of 6)