This part is very easy. There are 6 types of upgrades:

L- Laser gun, very high damage, but takes time to load. When you click rapidly, you can activate laser any time. Probably the best gun in the game

C- Bigger bullets. Little slow but high damage. 2 or 3 hits the bigger enemies.

H- Normal Gun. Do not take this if you want to trick the game.

S- Triple Bullets. Can be good but not better than laser and bigger bullets

Z- This upgrade activates 2 nearby turrets and lets them shoot randomly. Pretty useful.

M- Machine gun. Same as normal gun but has high fire rate.

Now lets get to trick the game. When you get an upgrade, your gun changes. So lets do that then.

1- Get any upgrade except laser or turrets (That will be L and Z)
2- When you get that upgrade wait for laser upgrade.
3- When laser upgrade comes, press the upgrade but do not leave your finger from the mouse. that will make the game faster and trick the game that you have infinite laser gun. you can move your mouse around and your invisible laser will kill all the ships.

This will help you to get the 15000 kill achievement way faster than usual. Thanks for reading :D