Keep every bomb u can, i can't quite kill the bastard yet but i can get 2 him with full
health and 14 bombs. I can destroy all the wing guns both the lasers and all the top guns
except the last cockpit gun, i must be getting close, i could do the same with 7 bombs
which im trying 2 do now and use the remaining 7 to carpet bomb the last gun whilst
concentrating continual fire.

The first answer has it right pretty much but one point worth mentioning.

The enemies do not fire when the bomber goes over but you do...

When the bomber comes position yourself in front if the boss to get the max out your bombs
and gun. This way you can do it with less bombs.

If you have 1000 coins spare (paid or won in game whatever ppl prefer) an invincibility
may help