If you bypass the launcher, you can start the game multiple times on the same PC. If each instance has a different character, you can join them into a single LAN squad to get get the multiplayer achievements.

It's very important to only do this with Steam in Offline mode. You run the risk of SHiFT servers banning you if you attempt this while online.

"Who You Gonna Call?" needs four players. The other co-op achievements only need two. To do this solo, you'll need up to four different characters. If you don't have enough, start the game and create some more.

There will be one main character, and up to three more secondary characters.

Optional: If you want things to be easy, even with the full-squad difficulty bump, play Sub-Level 13 in a lower game mode with a higher mode main character. That is, bring a True Vault Hunter into Normal mode, or an Ultimate Vault Hunter into Normal or TVH mode to run the mission. This requires some planning (don't run the mission until you're ready), or you can use a save editor like Gibbed to reset the mission status.

And since you're going to play four characters at once, set the game's graphics mode to Windowed, and reduce the resolution until you can see all four game windows without too much trouble. Smaller windows may also help with running the game multiple times on older hardware.

Save and quit.

Now put Steam into offline mode. In the "Steam" menu, "Go Offline..."

With Steam offline...

Find the program named BorderlandsPreSequel.exe. For me, it's in C:\Program Files (x86)\Steam\steamapps\common\BorderlandsPreSequel\Binaries\Win32

Every time you run BorderlandsPreSequel.exe, it will open a new instance of the game.

Start the game four times. If you set the game to low-resolution windowed mode, it'll look like this. Some useful options have been highlighted in each window.

If this is too hard to play, you might be able to increase the resolution in your main character's window once the squad is formed.

Choose a different character in each instance.

Have three of the characters join your main character in a LAN session.

You should now have a full squad. The three side characters will follow your main character to new areas, but they'll just stand around at the entrance. This is fine for "Who You Gonna Call?"

The other achievements will only need a two-character squad, but there will be more interaction between the characters.

With Steam offline...

Have your main character start the Sub-Level 13 mission, and get them to the Sub-Level 13 area. The quickest path is by fast traveling to Titan Robot Production Plant and going out the door to Titan Industrial Facility.

Have the other characters join your main character's session after they're at Sub-Level 13.

Leave the extra characters at Sub-Level 13's exit while your main character completes the mission. That's good enough for the achievement.

Once you're done, save and quit all game instances. Just to be safe, save your main character last.

Put Steam back into online mode.

Start the game normally, and play your main character normally to register the achievement with Steam.

Make sure Steam is offline.

Start two instances of the game, the same way as before:

• In one instance, select a high-level character. Ideally they should be a UVHM character playing in Normal mode, so they're vastly OP.
  
• In the other instance, choose a low-level character who can go into Fight For Your Life easily but who isn't so weak that they die immediately.
  
• Join them into a single LAN squad.

There are many ways to do this. The pattern: Find a convenient and quick way to send the low-level character into Fight For Your Life, and have the high-level character revive them with oxygen.

Have an oxygen source nearby that the high-level character can use to recharge. Vehicles work great and are portable, and if the low-level character accidentally dies, you can teleport them back to the vehicle.

The way I did it:

• Go to Janey Springs' place in Serenity's Waste.
  
• Have your OP character drive out to the lava pool nearby and clear out all the Kraggons.
  
• Send your low-level character to join them once it's safe.
  
• If your low-level character has an Oz Kit, have them take it off.
  
• Set the low-level character on fire by running them into the lava pool and back out. This should send them into FFYL within a few seconds.
  
• Have your high-level character revive them with oxygen.
  
• Recharge the high-level character's oxygen before doing it again. This also adds a little delay between FFYLs so the low-level character doesn't die too quickly.

Repeat 9 more times.

Once you've revived them 10 times, save and quit both games. Just to be safe, save and quit the main character last.

Take Steam back online.

Start the game normally, and play your main character normally to register the achievement with Steam.

With Steam offline...

Be sure your gameplay options are set to accept duel requests.

Set up a two-character squad.

Start a duel between them. If you've never done this before, have one of your characters melee the other. Then have the other accept the duel.

Kill the secondary character with the main one.

Save and quit both instances. Just to be safe, save the main character last.

Take Steam back online. Start and play the game normally to register the achievement with Steam.

Multiple game instances will work for FR4G-TP's "That Helped, Right?" achievement.

Once again, be sure Steam is in Offline mode before doing this. SHiFT doesn't like it when someone is logged in more than once.

Set up a two-character squad. FR4G-TP is the high-level character, plus one more. Do this in Normal Vault Hunter Mode, so the enemies don't kill your secondary character right away.

If necessary, repsec FR4G-TP to disable all optional action packages. Don't distract RNG with extra choices.

Everyone should turn off health regeneration. You'll need to get both characters to low health levels and keep them there until you roll the right VaultHunter.EXE modes.

Items and abilities that reduce cooldown time help.

Find a safe place near enemies you can aggro. I chose Flinder's Needle: Fast travel to Crisis Scar, then go out the door to Triton Flats. There are enemies just outside this room, and if you accidentally kill them you can aggro more on the mesa just below. They'll shoot at you from the doorway, but they won't go inside. It's safe to park low-health characters in the bullet shadow behind the big tube.

Pilot characters into some bullets until their health is low, then park them where they won't get hurt anymore. Repeat for both FR4G-TP and the second character. Now you have all the time you need to roll for Med Bot over and over.

Once you have that, repeat: Deplete FR4g-TP's health, and then park him in a safe place. Drive the second character into danger, and leave them there so they go into Fight For Your Life. Be sure you aim their guns away from the doorway in case you roll Funzerker. Once they're down, try to activate Senseless Sacrifice.

If you roll Rubber Ducky, you can manually revive your second character by rapidly slamming to stay within revive range.

Some guides say to put your crosshairs on the character you want to revive when activating FR4G-TP's action skill. I did it just to be safe, but I suspect it's not needed. Med Bot requires you to shoot your healing laser at the player you want to heal. Senseless Sacrifice is supposed to revive all downed players.