Don't use Orks but my IG play Necrons often. I suggest try and get into CC as quickly as possible as majority of Necrons are poor and your strength. If he takes alot of destroyers try and shoot these first as with good BS and weapon will quickly thin your numbers. Shame you don't have some ordanance weapons? Try and ignore the monolith if possible and concertrate on moving towards the enemy.
Avoid taking loads of vehicles/tanks as all Necron weapons have potential to blow them up (happens all the time for me!)