Narco Terror

If you are short on cash like I was, after finishing everything else in this gem of a game, there is a way to make the grind towards this final achievement a bit quicker.

Load up the first mission on Hardcore. BIG THING TO NOTE: CHOOSE THIS MISSION BY CHOOSING "SELECT MISSION"! If you don't, the game will reset your progress and start a completely new playthrough. Fight your way towards the section where you can get on the flatbeds with the mini-guns on them. Press to get on the turret. After that, make a manual save.

Now destroy everything in sight. You should get about $100 worth of money here. It might take you a while to locate where everything is the first time, but don't worry, I did this about 80 times to finish this game up, and you'll get quicker and quicker on this.

Here's the real time saver though, when you are done getting your $100, leave the turret: jump off of the flatbed truck, and get absolutely annihilated in no time by the goons that are still very prevalent.

The game will load your save as the checkpoint and you can rinse and repeat after that. Takes about 15 seconds max to get $100 each time. Buy the final upgrade(s) after getting the correct amount you still require, and hey presto, achievement!

There are four weapons (you pick them up as you go along) and each can be upgraded twice.

This requires cash which is earned by detonating red barrel and vehicles. Also, beating levels gives you cash too as does finding collectables.

To buy an upgrade use LT and the right stick to select your weapon and press 'A' to upgrade it.

You'll need over 15,000 (that achv pops before this one) and it took me over 1.5 playthroughs to get that much cash (and I was destroying everything).

This isn't a hard achv though, it's just a bit of a grind.

I personally found the first part of "Mission.5: The Betrayal" to be by far the easiest way to grind whatever money you're missing when the game is completed.
MAKE SURE YOU DON'T START A NEW GAME BUT INSTEAD USE THE "SELECT MISSION" TO CONTINUE,DOING SO WILL LET YOU KEEP YOUR MONEY!
I played alone on Hardcore & tried as much as possible to destroy all barrels & vehicles so when I was finished I think I needed around 3000$ to get the last upgrade or 2.
So in short everything that blows up gives you money but in "The Betrayal" you also get 50$ for each generator you shut off, so with that & the many vehicles & barrels there are in the beginning of this mission,you'll make around 8-900$ before reaching the plane where I chose to restart the mission by exiting to the menu & start the mission over & make sure that you ALWAYS start the mission from "MISSION SELECT"
In Total you're gonna need 19,000$ to get all the upgrades:

Pistol - 1st Upgrade = 500$ - 2nd Upgrade = 1200$ (Pistol Total: 1700$)

Shotgun - 1st Upgrade = 1000$ - 2nd Upgrade = 3000$ (Shotgun Total: 4000$)

Machine Gun - 1st Upgrade = 1500$ - 2nd Upgrade = 4000$ (Machine Gun Total: 5500$)

Minigun - 1st Upgrade = 1800$ - 2nd Upgrade = 6000$ (Minigun Total: 7800$)

Adding up to a Total of: 19,000$