Quickest way is to get a B-17 to pre-bomb the targets for you. Use the sicaly map with max player limit to get 16 bunkers. Two 2000lb bomb hits will leave a bunker right on the edge of blowing (Ive even blown one up with just the guns on the IL2m). You can then use your rockets to finish the job or just crash into it for the kill.
I'm half way to getting the mustang bombs after just two 20min games thanks to Squirell's B-17
He helped me get the target kill loadouts for the IL2m too.