Sunset Overdrive

These can be obtained by buying the collectible maps from Two-Hat Jack, and then collecting them in Sunset City. Once you have completed the story and challenges it is extremely hard to get any overcharge for it. You don't need the 100k weapon, I bought it and I struggled getting overcharge, so buy the maps first! If like me however, you are finding it hard to get overcharge later on in the game, this is the method I used to grind overcharge quite quickly to help me get the last maps I needed.

This setup will help speed things up, to get the most of your time:

Weapon - Hair Spray Bomb with SECOND AMENDMENT amp (this allows the weapon to keep regenerating ammo so you never have to use anything else)

Overdrives:

OD Killer
Fizzbot Killer
Electric damage (only use this if you use the Chance of Lightning Epic Amp)
Fire Damage
Sacrifice single shot (50% more damage with the hairspray bomb)

Load up Mission A heros duty and follow the video below :)

***NEW METHOD*** (CONFIRMED!)
There's a game braking glitch that will allow you to get this, and reload an older save afterwards to get all your OC back so you can buy guns instead of collectible maps!

Technically you could just pick up the same collectible over and over again, but to save time, here's what you want to do: while playing through the story, pick up every collectible you see, so by the end of the game you should only have a handful of every kind left. Then quit the game, go offline from Settings->Network, start the game up again, and go buy all the collectible maps you can. (dont worry if you cant afford all maps at once, just buy ALL maps of a certain type of collectible, and you can repeat this for the remaining ones) When you have collected them all and get your achievements, just quit the game again and delete the save from the console for Sunset Overdrive by clicking 'Manage Game' clicking the save game and choosing to delete 'From Console' ONLY. Go back online from the network settings, and load up the game and your previous save will be downloaded from the cloud with all your OC from before going offline so you can buy guns instead of these expensive maps!

This also works for upgrading 20 guns to level 5, and for the 50 gold medals!
but NOT for the 250 cloth items, as you have to own all of those at once.
(check respective solutions for more detailed information)

PS. My original solution was just a theory because i did this by accident, so this revised version is pretty much a copy paste of how D4 B1GD0G got this to work too, so a HUGE thanks to him for confirming this in the comments of my "I Like Them All" solution! :)