One of the big changes since I last played my Orks is that the sky can be filled with all kinds of danger and this requires some thought. The designed solution for Orks is the Tractor Kannon because it comes with Skyfire, Tracktor.
18 points buys you a Mek Gun with 2 Gretchin. The Mek Gun has T7,W2 and the Gretchin have a BS of 3. Yes folks - I said 3. They can shoot!!! Well I never. The Mek Gun can be upgraded to a Tractor Kannon for a further 12 points. And there is the option to take additional gretchin and Ammo Runts for re-rolls. The Tractor Kannon is S8, AP3, Heavy 1, Skyfire, Tractor. You may have 5 of these in a squad.
Skyfire allows you to shoot at flying vehicles using your normal BS in this case 3 so you need 4+ To Hit. Ammo runts allow re-rolls. That should average out 3 Hits at S8.
The down side is that you can only Snap Shot at non flying Units which, for an Ork who would normally need 5+ To Hit, means you would now need a 6+.
Tractor means that the flyer is automatically Immobilised when a Glancing or Penetrating Hit is scored. A further dice is rolled and on a 1 or 2 the flyer will immediately Crash & Burn!! The flyer is torn apart and it scatters 2D6. A Large Blast template is used to see if anyone underneath is injured. If covered by the template then you take a S6, AP- hit.
Interestingly there is almost a natural solution in that the Ork Ballistic Skill (BS) is so bad that you tend to make up for it with volume. The Ork BS of 2 means that you need 5+ To Hit. Well if shooting at flying vehicles you need a 6+ To Hit.
Consider the Lootas.
They are armed with Deff Guns, S7, AP4, Heavy D3. A decent squad of these guys will be laying down a lot of fire power. A squad of 10 (and there could be a lot more) would be expecting to generate 15 shots a turn looking for 6+ To Hit. There should be a couple of Hits in there and more with a bit of luck. The bigger the squad the higher the number of shots.
This principle also applies to a lot of other units too - Buggies with Twinlinked Rokkits, The Flash Gits etc etc.