There are several videos on youtube soving you how to do it. (Including mine, quite dull boring one. https://www.youtube.com/watch?v=7Xk3aIk-a6A
The Key point is ALWAYS to NOT fight the boss. Just late attack the territory that it is standing on. It may take a few tires (particularly for gold star) but the level is actually quite beatable.
Just, don't focus on the boss at all. Just clean up after it.