The answer is always take more vendettas...seriously take more.
Having said that lascannons, melta, and plasma are the
IG MC answers. Sniper rifles of any kind just don't work in the
IG dex. Don't forget to use
BiD against the
MCs.
LRBT are great against wraithguard. That or I really like demolishers or medusas. These are wound on a 2+ and ignore the armour save of wraithguard so they work pretty great. Battle cannons are better in this case as you can stay at the back corner and be immune to most of his short range fire power.
Don't forget you can tie his wraithlords up in
CC forever with just a simple stubborn/fearless IS. The thing will kill ~2
GEQ an assault phase. (so you should stay for challenge+8 guardsmen = 5 assault phases with a
LC somewhere nearby)
Feel free to assault the wraithknight if he is foolish enough to allow it. If you bring melta bombs he will regret it. Otherwise that is going to be a real nightmare I would not recommend trying to kill it. Just plan to loose ~5 vehicles to it over the course of the game and try to get some cover to reduce the damage. (with cover and the short range you can reduce the damage to 2-3 chimeras but don't let it charge vehicle whatever you do).
The scythe wraithguard and double sword are the only wraithguard able to deal with
GEQ in a timely fashion. They are all short range and if he walks then you should be able to dictate what the wraithguard get to fire at.
Overall just remember that he will be very short on models and most of these have short range weapons so make him spread thin and walk across the board so you can waste his offensive output. Plasma is great as they can just strafe the wraithguard while popping them with plasma shots. You will however need something on the lines of lascannons or melta for the wraithlords.