Here is an army list sent in for feedback.
Necrons - Primary Detachment
Overlord - Catacomb Command Barge
2x Nightmare Cryptek (each attached to a Deathmark unit)
2x 15x Warrior
9x Warrior - Ghost Ark
2x 5x Deathmark - Night Scythe
3x Annihilation Barge
15x Canoptek Scarab
3x Canoptek Wraith
Grey Knights Allied Detachment
2x 10 man Strike squad - Razorback
Here are my thoughts... Not sure how many points (2000?) is this list so keep that in mind.
- Necrons -
1) The Destroyer Lord should have Mind Shackle Scarabs and Weave. He is a real nightmare for many armies. I am supposing you plan to attach him to the Wraiths... Might as well equip them with Whip Coils. I'd try to find the points to bump the Wraiths up to four strong.
2) The Overlord should have a Warscythe and Mind Shackle Scarabs... That's all he needs.
3) I wasn't aware you can field a starting swarm of Scarabs over 10 strong. I run eight and they do just fine. Five on the charge can drop a land raider.
4) The big blocks of Warriors are awesome. They can do a lot of damage to most enemy units that move into double tap range. You've got lots of big scoring units and that's a good thing.
5) I'm a big fan of the Doom Scythe - people absolutely hate it. It's a huge fire magnet and does double duty shooting at enemy flyers with the Tesla destructors and can nuke any ground unit with its Death Ray. I'd drop one squad of Deathmarks and a Barge to fit one into your list.
- Grey Knights -
1) Coteaz is the man !
2) I'd drop both razorbacks as they give up First Blood very easy. Each Strike squad should have two psycannons. You might also want to include a daemonhammer in each unit.
You've got lots of midrange firepower. The flyers can cover the rest of the table.
So you've got to decide whose your Warlord. I run a similar configuration for my Necron HQ... So far I tend to prefer using the Overlord in the CCB for my Warlord... It can be situational though.
The Scarab swarm can do lots of great things for you and are the answer to land raiders... Keep them protected so they don't get decimated by blast and templates. You can use them as a screen versus fast enemy dedicated melee units to buy you another turn of shooting. The Scarabs can also tarpit enemy units such as IG blobs.
With Coteaz and two Strike squads you've got lots of anti drop pod defense as well as versus daemons... We've Been Expecting You and Warp Quake. It's a very solid combination for sure.
I'm sure you'll find this is a strong list in the current meta. You've got lots and lots of firepower plus some decent melee.
Happy hunting !