Protip: NEVER go in the bunker in the middle of Wasteland. EVER. It is a fucking meatgrinder and you will eat grenades at an alarming rate. I haven't been in that bunker for weeks. They could have set up a flatscreen and stripper pole in there and I would have no idea.

Slap on that loadout I listed above (including Predator-->Harrier-->Chopper Gunner) and just sprint around the farthest edges of the maps, where the radiation markers are. Peek out around houses, bushes and walls and find the snipers (because there WILL be snipers). Pick them off with the UMP45. When you pass a mounted machine gun, don't be afraid to hop on and scan the field with it, but don't set up shop for more than 10 seconds.

If you just skirt the edges with a silenced UMP and Cold Blooded, getting 5 kills for a Predator is cake. Find a wall to go prone behind or crouch behind a barrel near one of the houses by the chopper. Whatever you do, TAKE COVER and then let 'er rip with the Predator.

That will usually get you to the Harrier, which on Wasteland is almost guaranteed to get you to 11 kills for the Doomsday chopper. I only just started using Chopper Gunner, and in 3 matches on Wasteland I've had 30+ kills each time, and it wasn't particularly difficult. Enjoy.