Maximum Stopping Power - 20G (Original Achievement)
Difficulty: 2/10
To get this Achievement, you simply need to fully Upgrade 1 weapon.
If you have roughly $50,000 you'll have no problem.
First head to the nearest Friendly Fire (Gun Store), then open the shop menu and select Upgrade. I suggest upgrading the Baseball Bat as it is the cheapest besides out of the way hidden weapons. Once all of the slots have been bought in each area, the Achievement will unlock.