You cant have 2 lash of despairs on the same model.Issue here is that you have no real board control and most of your attack is dependent on good psychic rolls. If you want to keep this kind of list I'd switch out one of the tzeentchi princes for a Slaaneshi one and give it two lashes of despair and three psyker levels in biomancy so you can iron arm yourself and have 4d6 Str 9 AP- Rending shots at 12" that can be skyfire or not (if he's in the air as he should be). Still only one shot at one target but almost guarantees a unit is being deleted per turn.
If you're really wanting FMC's out of daemons you might as well do an infernal tetrad and then run a small CAD (or if you have the models one of the big formations). A tallyband made of nurglings is ridiculously cheap (in game at least).
Hope that helped!