This achievement is not unlocking for some players. The fix seems to be to uninstall the game, and start over from scratch again.
There are 5 different upgrades to max out. Light Guns, Heavy Guns, Laser Towers, Defensive Cities, and Overdrive. They cost 10, 50, 100, 200 upgrade points to max out. This means you will need a total of 1800 points to max out everything. The best way to earn credits is to get as far as you can in Arcade Mode. Upgrade the Light Guns first. The best way to play the game is when you have a lot of Light Guns, start the two points for firing and cross them in an X-form. When fully upgraded, you can literally just sit there and move through waves without even looking at the screen. This will help you get the credits for everything else faster.
Easy Method: Play Survival Mode. Put your finger down near the center bottom of the screen. It will be an X-shape of firepower which will protect you as well as kill pretty much everything that comes by. You will get upgrade points the fastest this way.