Hillbilly Doomsday (EP)

I found out that when you beat a boss, and go to the store (with the old man), if you buy an item related to an achievement, you can quit out immediately after the cheevo pops, load the game again, hit Continue and you will have the same ammount of money that you had Before the purchase, every time.

2nd pistol...........500$
Micro SMS........1000$
Flamethrower..1000$
Shotgun.............1500$
Minigun.............2500$
Magnet...............500$

Do NOT buy the RPG (5000$) , you'll get it for free, during the 5th boss battle.

With only 1700$ , using this method i got:
2nd pistol+ Micro SMS+ Flamethrower+ Shotgun+ Magnet achievements.
I quited out, loaded the game back up and still had the 1700$ and saved it for the Minigun (the best gun in the game IMO).

With this method i got the Minigun and all the weapon achievements early in the game, and was able to get the Heavy Armor and extra Lifes/Energy, this made the game much easier.

The Minigun is pretty OP in this game. It has decent spread range, and a very high fire rate. This one costs $2500. You get to the shop after each boss. You can also quit out after getting the achievement and reload your save to get the money back if you so wish.