Just as achievement description says - you need to buy every weapon (5 primary, 5 secondary and 4 specials) and fully upgrade them.
You will need around 1-1.2 million credits for that.
The credits can be easily grinded on the later missions. By the end of game (just before the 'final' fight) you could get a maximum of 50600 credits per 'Defend the base and soldiers' mission (100% completion, maximum reputation).
Just keep finishing those missions and you will get the needed amount of money. Even if you have no 'Defend the base' missions (yellow dots) on your map, tap the 'Sign out' and then load your save again. Keep doing that and the defend mission will randomly appear.
Or you can 'finish' the game (see the 'Hero' description for details) and get the bigger list of available missions with yellow dots guaranteed.
The 'Defend the base' mission has no its own achievement but it is a great help in grinding money and experience. It will always give you at least 30000 credits and 2500-3500 EXP.
The goal is to keep all of your soldiers alive while surviving a certain amount of waves. There are always W+1 soldiers, where the W is the waves count. The higher protagonist level is the more waves are. By the last 21st level you will have 6 waves and 7 soldiers to be kept alive.
The tactics for this type of mission (6 wave type) is pretty simple. There are always three entries from which the enemies are attacking.
1. Deploy the droids near two of the entries in front of the soldiers (so they will get most of the frags) at the wave 1. Move protagonist to the 3rd entry and kill all the attackers only there, leaving the spot to help droids sometimes if necessary. Do that until wave 3.
2. Put the sentry gun on the 3rd entry (where you previously were), run around all of three entries keeping enemies far from your precious soldiers (do not forget - 100% completion, 7/7 soldiers alive).
4. Put the second sentry gun on the needed entry (you should see now which one is weak) at the end of wave 4.
5. Put the last sentry gun at the end of wave 5.
6. Do not forget to use defectant gas and missle drones. But deploy your missle drones wisely as while they are deployed you cannot use any other special weapons and ought to wait until it spend it's ammo and fly away.
Keeping that order will allow turrets fire even if they are out of ammo (that's a glitch, only one needs to be the turned on to keep other two alive).
Just try not to push the turrets as they will infinitely slide out of the map (bug).