My plan is to go aggressive upfield with my tyrant and crone, screened by the gargoyles and wait for the mawloc to drop. Meanwhile the warriors venom and zoanthrope herd the gaunts and fexes to the midfield while venom terv spawns and nurses the biovore.
I took the wars for extra synapse and pinning gun not to fill points.
Necrons are not known for there strength 8 wps and hopefully I will avoid his doomsday barge.
I plan both Nids and crons and I can tell you right now your MC's are perfect fodder for his night scythes and his annihilation barges if he runs them.
My old room mate played nids as well so I played crons against nids many many times. I can tell you that your number one answer for crons is pure numbers. I would say run as many tervigons and biovores as you can field. The AP4 from your biovores will wreck any warriors in open field, your tervigons can "can-open" his vehicles since they hit rear armor, and with a weight of bodies that 3 tervigons can put out you should have no issue flooding the field.
It might be useful to post what his army lists usually consist of.
Iirc the majority of necron vehicles have effectively AV:13 so until you have penned them once the dakkafexes might not be able to shoot anything and same with the mawloc..
Oc if your opponent mostly run necrons on foot there wont be any problems.. if he runs a vehicle heavy list you will have trouble hurting him at range..