I tabled crons in turn 3 once with my Zhadsnark and Bullyboyz list. But i got very very lucky on my charge rolls..
I do play Crons as well.. And yes i am one of those players that says Necrons lack mobility,.. in general...
With that i mean the bulk of the Necron force
ofc.. Blades/Flyers Wraiths and Scarabs are mobile enough. But those arent realy the units that you need nor want at your objectives.. They all go down quick,.. except wraiths.
Anyways,...
Drops any battlewagon you got and just go for trukks. Thats 3 trukks you can buy instead of a B wagon. This way Gauss has less of an impact.
Bikers going down to Tomb Blades? I dont see how.. Yes blades can ignore cover... boohoo.. still a 4+ Armour save against AP5 gauss. Bikers are fast! And they hurt cron infantry realy hard. I have had Zhadsnark go toe to toe with wraiths multiple times and to date Zhadsnark has prevailed more times than the Wraiths. His squadmates are less lucky though

Just drop a painboy with a bike in that unit!
Lootas tear through Cron infantry like Tau cuts through anything.
Dakkajets are failry good against cron infantry as well. Against vehicles not so much
ofc but hey that is where powerklaws come in
ofc
I have played against 3 Necron players with Orks and i must say... I dont find Crons that hard to play against with Orks.. Dark Eldar, Tau, Astra Militarum, Eldar en Tyranids and the occasional Daemon(kin) list are a whole other story... that is a uphill fight that makes you hate the Ork dex... I rarely win against those...