Well I've never played nor fighted necrons so I can't really say how you should be playing them, but I play Orks so I can tell you how to expoilt their weakness.

It really depends on the list your enemy is bringing, but you probably have the army balance in your favour as Orks have a pretty bad codex.

Orks have a shitty morale, and with Mob rule.being as silly as it is you can destroy a 10 boyz+Nob squad in one turn of shooting if you make them fail their morale. The Nob will do the job for you at killing boyz, and if the Nob dies they will probably be runnning away.

Orks relly on close combat, so wrecking as much vehicles as you can to slow them and get more shooting turns before they can get in choppy range is the way to go. Also Orks vehicles explode very easily, and adding the explosion to the Mob rule you can hurt really bad a unit just by exploding their vehicle.

You will loose in CC almost for sure, so use your scarabs (are those the little swarm guys with four wounds or so?) To tie down problematic units and give yourself more time to kill other things, (Maybe just run away if you can. Distance is your friend.). Don't use your focused CC units (lychguard? irc they got a involve right? If so these are the guys who want to try and take out PK) on boyz, they are elite wreckers due to the bucket-of-dice effect and tons of wounds to go through.

TL;DR: Keep the distance, focus on delaying their charges by destroying their mobility and make them fail as much morale as possibile. Asume you will loose CC, and be ready to sacrifice a unit without remorse if that means you are getting an extra turn of shooting.

That's just a basic guide against orks, but you never know when orks will have a good day, and if so you are for sure doomed. That's what I love about them!

Also I'm on mobile so as I'm typing this I can't see what I'm forgeting, feel free to ask anything you want.