Sunset Overdrive

For this achievement you will need to upgrade 10 of your guns to level 5. You need to play through most of the story before you can unlock all guns and even then you need a good amount of Overcharge to purchase them from Two Hat Jack. Some weapons unlock from missions.

Purchased Weapons:
Freeze Bomb: 5,500
Acid Sprinkler: 7,500
Captain Ahab: 9,000
The Dude: 9,500
One Handed Dragon: 10,000
Taunt Bot: 10,000
The Roman Candle: 15,000
Shorty Shotty: 20,000
Proximity Mine: 20,000
Turret Copter: 25,000
Fizzbot Rifle: 40,000
Hair Spray Bomb: 55,000
The Shocker: 70,000
Charge Beam: 100,000

Campaign Weapons:
AK-FU
Dirty Harry
Flaming Compensator
High Fidelity
Murderang
Pulse Mine
TNTeddy

DLC Weapons:
AK-FOFF (variant of AK-FU)
Head-Banger Gun (variant of The Dude)
Hotty-Shotty (variant of Shorty Shotty)
Nothin' But The Hits (variant of High Fidelity)

I've found a mission that's good for farming OD kills called "Getting The Band Back Together". You have to play through until the part when you need to help your drummer. Don't follow the instructions but keep bouncing on a drum. The OD constantly swarm you so you can just bounce and then shoot from any gun.

You can replay this mission from the Log in the pause menu. Note that the mission happens later in the game when you've met Las Catrinas.

Of course I would recommend playing the game and getting every other achievement before resorting to doing this.

Note that there is a weapon amp called "Mine All Mine" which gives a chance of a kill dropping additional Overcharge.

Onsidic has some additional tips:
- Don't forget about the Overachiever amp for weapons that increases the XP level up for the weapon it is attached too. - *season pass content or free from spin in Chaos Squad*
- Combine this with skills that give you increased ammo and/or damage output for those weapons, should make leveling up these a lot faster.

More Info
Now that I have the 20 guns achievement I know this method is most effective when at least four weapons are leveled up together. On my final run I was finishing the Fizzbot Rifle, Charge Beam and Taunt Bot (all level 4) and I just acquired The Dude and Proximity Mine so they were from level 1. Basically as you use one weapon you will most likely have ammo dropped for your other weapons so once you run out just switch to another weapon and continue on.

Overcharge Strategy
I have still not found a good way to get Overcharge. Chaos Squad does not guarantee it as I played in three games in a row and only got clothes, money and weapons (which does not count towards this achievement) on the spins. After completing all missions, side quests and challenges I did have enough to buy every weapon and the majority of collectible maps but not enough to buy the most expensive ones in Downtown (10,000 each).

I did avoid buying certain maps like the High Points and the Eavesdropping ones because it was just as handy to use a video guide. I also avoided buying the more expensive maps for Tagging. machinimawingman on YouTube has good, straight to the point location guides for those three items.

Which Weapons...
If you get a DLC weapon you can avoid buying Captain Ahab and the Charge Beam (as https://www.trueachievements.com/SerjiTank.htm recommends).

If you're forced to use one of these weapons....the Charge Beam is decent enough at leveling up but you will most likely have an Overcharge problem. If you want to go with Captain Ahab, Calex dEUS has this strategy:

I found the Ahab to be as quick if not quicker than some of the others. Trick is ammo all you have to do is stand near one of the Two Hat Jack Stores on the street , not in a base.

Costs 50 OC to refill ammo , you will easily make this back from killing OD, once you run out of ammo refill from jack and shoot some more OD, the reason it's so slow in the band mission is it takes ages for the ammo to spawn.

Oh I should add also once all the OC are dead move to the next gun on the map , I did this on the East off the map between the 3 shops there and got this in less than 20 minutes.

LitaOsiris's solution is on the money, but I want to point out another place you can grind this out (grind!! get it??!)

Simply make sure you have some low level weapons on you, and then replay the mission, "Bryllcream" - at the end when you have to free him from the truck for the first time, you get jumped by some scabs, kill 'em off, and then the truck rolls away. DON'T FOLLOW THE TRUCK! Now, just keep grinding around the edges killing OD's that stream in. You'll get ammo and health pickups as you shoot. If you run out of ammo for one weapon, just switch to another. You'll pick up ammo constantly.
One nice thing about this level is you don't need to think much as you're grinding, you just pick a weapon and shoot!

Here's a video: http://1drv.ms/1uaiW0S

If you have not already got these, you can work on these too:

and this

a zillion of these

and this

this

this

and, finally, this

i found it easy on BUCK STOPS HERE challenge ... after you finish the first three tasks .. the flying monster will show up ... there is a window that the OD's keep spawn from it ... for almost 2 minutes you just keep shooting between the 2 cars in the middle while the OD's are spawning ... you should have the THE SECOND AMENDMENT amp it's really helpful here :) ... i hope this help