You must obtain gold medals in all missions to get this achievement if you can't beat it skip and save it for later you can return to the SkyForce Barracks later and replay those missions.
After playing about a little more then half way through the game you should have all level 3 airplane parts (if you've been earning gold medals) After equipping them in the hanger you can go back and play your earlier missions that you didn't do so well on it also helps if you have separate planes tuned for specific mission types I have listed the tune parts that i used for the specific missions but you may want to customize it to fit your own play style.
(i don't know what there exactly called but I'll described what they do)
Dog fights:
1. Increased Machine gun damage
2. Faster missle reloads
3. Increases Orbital strike laser time
Races:
1. Longer boost & Longer Super Boost
2. Allows better Turning while boosting
3. Increased Machine gun damage