Hey there!
Unless you're totally set on mono tzeentch, I'd suggest adding 10 plague bearers for backfield objective holding and 3-4 beasts of Nurgle as a distraction/threat.
I use beasts of Nurgle and they very rarely let me down. Tough as nails and fast. I use these alternative models to represent them:
http://titan-forge.com/fantasy_miniatures_daemonic_kingdom_scourge_set
The other dependable unit for me is a
GUO - level 3, either 2*greater reward or 1*greater and 1*exalted. This dude is always tough to kill and with some lucky rolling can be impossible to kill, I often run him as my warlord.
Soul grinder with mark of Nurgle are tough as nails.
Daemon princes with mark of slaanesh, lash of torment, wings and biomancy are pretty decent at attacking air and ground units. With some luck they could have an assault
2D6 str 9
rng 12 weapon.
15-20 daemonettes are capable of killing pretty much anything. I use mine to attack Terminators, dreadnoughts etc. You need to take a lot of them in order for them to be effective. I never deepstrike them and instead run them next to the beasts, with their additional run moves they're almost as fast and generally get a turn 3 charge off.
Remember when using Daemons of nurgle that models with 8inches provide +1 cover to them since they're armed with defensive grenades.
My suggestions.
Crispy