HINT: You can save a lot of energy and time if you refresh your browser at the correct times. For training, reset the browser if you miss on the 1st attempt. When the game loads again, you are back with the same amount of energy that you had before training. Saved a lot of cash and redos by this.
You are better off first upgrading the engine, wheels, and gearbox to make your car go farther. Gas upgrades just add "more time" since you deplete gas in the air and in slow motion. Also, you can use the "Like" double money bonus in the second round to get a bigger cash reward since you lose your upgraded car in the next rounds. I found the gun of little use (you get some $ for kills, but hitting zombies is inevitable), so I upgraded it last.
Even with learning to chain combos and everything, still took ~80 flights (with ~5 crashes) to get to all upgrades and the flying monkey. For quick money early on, do s 360 spin if your launch is good.
Melee: short range, I think it uses less oxygen. Harpoon: long range and more O2 consumption. With the power upgrades, they would slice through every fish. Harpoon is good if you line up the fish. You also get the most $ for cutting the tips off fish and wait until they bleed. Always dive deeper since bigger fish=more $.
All about doing damage and dodging. Get the axe mastery and dagger mastery skills maxed out. The Zombie Ripper (axe) will do huge damage and poisons mobs. The Mermaid Tears (katar) will attack really fast and not allow them to get close and it slows mobs.
You could complete the boss run with taking damage. If you have full health at the end of the last boss, you'll get teh badge. The game only checks your final health; it does not track your damage.