Fighting Necrons

I lost my last three games against this Necron army (changed a little bit due to points played)

- 2 high lords + 5 Lords + 2 Krypteks with all kinds of equipment like those swords, regeneration-spheres, re-rolling dice, 3 mind-controlling scarabs, veil of darkness.
- 2x 6 Wraiths
- 3x Annihilation-Barges
- 3x 5 Immortals + 2 Kryptek
- 8 Scarab-bases
- Tetrach-Walker with Multimelta

The first two turns I shoot the crap out of his army, killiing most of his Immortal and his Scarabs, dropping most of his Vehicles - but in turn 3, his Wraiths start to chew thru y Army because I am not able to kill enough of them before.

Is there a trick I missed? What are the best units or tactics to kill/bind these three units.

It sounds stupid, but drown them in splinter shots. They have a 3++ so don't bother using darklight weaponry. If they don't sport whip coils you could tarpit them with wyches, but that's a. very unlikely and b. not very effective. See them as the Necron equivalent to Hammernators, but faster and sporting more wounds.

The Wraiths sport 3 whip coils each unit.

I usualy field something like that:
Blaster-archon
2 Haems
4 Blasterborn in venom
2x 5 Blasterwarriors in venom
5 wyches in venom
9 wyches in a raider
1-2x 10 Warriors with SC
6 RJB with 2 heatlance
2x 5 Scourges
3x Ravagers
Aegis defence line with quadgun

I try to shoot the hell out of his Wraith - but I have to focus on three different threats at one time. And those Wraiths are fast as hell.

But you are right, i tried to instakill those wraiths with my blasters - seems like I have to switch target priority

for splinter fire a Wraith isn't harder to kill than 2 Marines. Darklight kills them outright but since they get a save against that you should focus on blowing up their vehicles first. Engaging them in close combat is a bad idea for most of out units due to whip coils and sigh strenght, but Beastmasters should do ok against them. Whip coils only work on models base to base, and beastmasters have large aenough bases that most of the models can stay out of base contact. Chimerae also have 4++ save so they aren't killed that easily.