Ratchet & Clank: All 4 One

While UlvenFenrir's solution is great and probably the fastest way to earn bolts, I'm playing nearly 2 years later, and looking for people to play with is pretty much hopeless.

So I found an easy way to farm myriad bolts by yourself by using four controllers (note that if you have a PS4 you could try using those controllers, as the extra players won't really need to be played by my method). Also note that you can do the same with three controllers but you will obviously get less bolts. It just earns a lot more than playing solo, it's dead easy, and means you're not reliant on boosting sessions/matchmaking.

As a bonus, it also doesn't involve any fighting, so you can even use it to gather quick bolts with your unused characters, which will make quick work of buying their character specific abilities, which are required for trophies (and any other weapons, which are not).

Unfortunately like UlvenFenrir's solution this also relies on you being very near the end of the game, but some of the tricks may also be applicable to earlier levels.

Load up Location 9 - Uzo City, Segment 2 - City.
I did it in Online - Host Game - Joinable Friends (Only). You can probably do it in an offline lobby but I never tried. You can compare the numbers to see if you get less or not.
Difficulty is Normal as I don't think you get any extra from Hard, but you're welcome to try, then leave a comment.

Once you're in the lobby simply select the character you want to use and start (as stated above it can even be your unused ones, but if you're going for the one million bolts I believe it must be on one character). You should load up in an area with a hookshot/grapple target in front, ammo pad on the right and a few crates on the left.

Ignore everything (the payout is pitiful when alone, and it's not worth the effort to spawn the other players yet) and just run straight down (towards the screen). As you pass some more crates, you will need to turn to your left, and the screen will turn also. Keep running downwards until you reach a metal jetty, and this is your destination.

You'll want to stand just below the yellow and red metal crate, and then (now make sure the three extra controllers are turned on) press X twice on all your extra controllers so they all spawn in next to you. You now have a four-player lobby.

Walk to your right where you will see a Bolt Grabber crate, break it and then go on a rampage on this platform: first break the bolt crates behind it, and move around from right to left, breaking the tough crate as well as one last pile to the left of the ammo.

After you're done simply quit out (it will save your bolts!) and load up again. Note that in the top right you can check how much you've earnt this session, and if you tap L2 you can see how much you have in total.

All in all you should get just under 20,000, which is not bad for a 30-second blind-run which you can mostly do while multitasking. Overall the whole loop including load screens takes under 2 minutes, but since so little of that requires any real attention you can easily multi-task to make better use of your time- I would recommend reading as it is much more stop-start than videos.

I timed myself to get 500,000 -I think it took half an hour- with this method, so it's an easy and consistent path to a million and to character upgrades also. Hope this helps and leave any comments if you find something out.

easiest way to do this is to fully upgrade your Critter Strike, Warmonger, Plasmabomb Launcher and Thundersmack.

play online and pick the chapter Uzo City and play the first level City Ruins and the 2nd level City. as soon as the third level starts, quit out and repeat. doing this with the Critter Strike helps alot because it turns enemies into hogs very fast and yields a bunch of bolts as well as co-op points. using Plasmabomb Launcher for the 1-hit kill turrets is extremely useful too. use the Warmonger when you run out of ammo for the Critter Strike and use the Thundersmack to destroy crates faster and gaining bolts.

there is also a few x2 blue bolt crates that you can destroy to get a bolt multiplier which can gain you quite a good amount of bolts. just run into the stacks of creates nearby, you will destroy them automatically.

lastly, you want to play with at least 2 other people so that you are 3 in total because the more players you are, the more bolts you earn. you can earn anywhere from 100,000 - 150,000 bolts per run. takes 15-20 minutes.

simple trophy, but a little time consuming. not too bad overall.

if you need more help, send me a pm on psn or on here.