Have been playing this game of late. What I find the problem about this game is that it is too difficult. You barely are able to finish the first stage of the game. Then most of the time u are probably like grinding the first stage if you are aiming for the crystals. Then also comes the annoying part where it keeps play the advertisement when you are in the main menu. Also at the end of the game. Then also the part where you actually need to restart the game to get back your 2 plane lives. It seems that the shield upgrades do remain. Also the amount of crystal your shield generates is according to the number of missiles on screen. Hmm also some rare occasions I think the boss comes out some random skill if you start dragging the battle too long and it becomes somewhat more difficult.
Explanation of the planes
A good front assault plane I was using Tech F-11 for up to level 6
Its a good homing missile plane. If you are always dodging missiles then this plane is for you because it attacks from the side. What I find hard about this, is that it lacks firepower usually
This plane has a small front attack then Tech F-11 but it’s frontal attack is more powerful then Tech F-11
Recommended upgrade strategy for the game.
If you get the bomb upgrades. Keep them. As they are very useful for boss fights. Shields are pretty useless because once you get hit you use a shield. Also do not use revive as it seems to waste 2000 crystal. It doesn’t seem to describe this anywhere though. Also gaining score gets you to trophy and they will give u some crystals to use and things like support+1. My recommended upgrades are increase bonus diamond. It comes very helpful as you will gain more diamonds per plane kill.