Agreed on this comment above. Even our spells with CV of 5-6 will have difficulty. However, it might be worth running your AAR with both Dermal Robe and Dark Wizardy for +2 to cast/unbind. You will at least get off Ferocious hunger spell which can be game breaker in the right use.... It could effectively double the output of GKoTG when fishing for their 6 MW on 6's to hit and doubles the Horrors output if you roll a 5/6 on the D3 roll.
I also agree that your horrors might be the lynchpin against tzeentch. Since they have no rend they are only good against 5/6+ saves. Which is the majority of tzeentch army! Buff them and move them twice with Royal Mordants, charge and delete the flamers turn 1. That is kind of necessary to survive. If those flamers are allowed to shoot for a turn, I don't think you will win that battle. They will eliminate whoever they shoot...
What if you ran a hollowmourne faction instead of blisterskin? You can still employ broad use of flayers. They will also get to reroll 1's to wound fon the charge in Hollowmourne. Throw in a royal mordants battalion, shoot double battalion if you want. Ghouls will simply fall so might as well employ all horrors / flayers / TG or ZD.