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I mean friggin evil nurgle.
Units of 40 all with Heralds with that stupid Miasmia of Pestilience and uses that fat special character on the chair to top it off (Epidemeus i think)
I use pure Slaanesh so fear isnt the problem, its me being 2's in everything and no matter what I never have enought dispel dice to do anything, AND he has 1 unit of something else that makes me -2 to casting.
TIME MUST COME TO STEP UP AND TAKE BACK WHAT YOU TOOK FROM ME!!
Tzeentch heavy magic. Between flickering fire and infernal gateway you should do enough to harm his blocks to then move in with chaos knights. Manipulate his slow moving force by march blocking with disc riders or marauder horsemen.
Banner of sundering is annoying but you can offset it by clusting your casters around a skull of katam, so the +1 from the skull and being Tzeentch balances the -2. Not a brilliant answer but still a solution. Then just focus fire the unit the standard is in.