After level 1 and 2, upgrade your minigun once each. After level 3, you should have around 2000-ish money, so I upgrade turret and movement speed once, and max out my minigun. After level 4, I but and upgrade the shotgun once, and buy ammo. In level 5, it is possible to beat it if you get the freeze power from the first crate, but if you don´t and die, go back and upgrade armor. Level 6 & 7 are possible to beat with just the shotgun, but 7 can be tough if you don´t get at least one freeze/heal. If you dont have enough for the rocket before 8, do a quick run to die in it early for a bit of money. Use the rocket to go through 8 and 9, and tunnel through 10 with the rocket, try to pick up ammo so you don´t have to use the shotgun at all. Use the rocket for the last levels, and start at the top right for the last level and go clockwise to save having to enter the room with just a barrel in it, it´s a bit faster. My first time doing a guide like this, so sorry if it´s bad/hard to understand.