Gears 5

Even though it says no "dupes" you CAN still use a guest controller. I unlocked this doing split-screen on my console with a guest controller and a friend online. All three were set on Beginner difficulty to speed the play along. We completed in one sitting without breaks, except as described below in Note #2.
Note #1: as we played through it did NOT update my achievement percentage but still unlocked for me and my online friend when we finished the DLC campaign.
Note #2: as we played through my online friend was twice "kicked" from the game. When this occurred I paused the game, re-invited the online friend, and once he rejoined I restarted from the last/ prior checkpoint to ensure there were no gaps that might have been saved with only two player co-op recorded.

If you have another 2 controllers, you can do it playing alone (on Beginner). You need to maintain the 3 controllers near and it's a good idea if you know which controller is which character.

I used Lahni, but Keegan is a good option too. Anyway, you can use the rest of the team for certain occasions (Lahni for Juvies, Keegan for bosses and Mac for dealing with bullets).

If you aren't playing as Keegan, remember to go near him and activate his ultimate (or if he is far away and you don't have ammo, switch to the other player). I used Lancer, Lancer GL or Hammerburst and Torque Bow/Boomshot

Campaign on Beginner is easy, even playing alone. I died on the following sections:

Chapter 2 - The Carrier:

Just move the 2 dummy accounts as far as you can (I recommend taking the controller and go up the stairs, the same for the other account). If you tried to move dummy accounts without pressing A (or whatever button you have mapped for roadie run), it would be too slow and main account will die 1 hit KO from Carrier.

Now that your accounts are "safe", use your main account and "lure" the Carrier away from your dummy accounts. Try to kill every Juvie, but you have to keep an eye on Juvies attacking dummy accounts. You can go and kill them or just take one of the other controllers and kill the Juvie(s) as one of your dummy accounts.

Just keep firing explosives and/or grenades to the Carrier and just keep close so he focus only on you and not on dummy accounts.


Chapter 2 - Venom run:

Just focus on your main account. Forget about dummies. They will endure the toxin/venom as long as you know the path to the exit.

Chapter 6 - The boat part

Put the 2 dummy accounts back of the boat. Incendiary grenade Juvie can kill one of your accounts if they stay in the fire.

The rest are tips if you are getting trouble in some parts:

On Beginner, you can revive yourself as long as you don't receive a good amount of damage. Getting on fire (the lava that falls on Chapter 3, Incendiary grenades and the toxic from Wakaatu) while DBNO is almost a certain death.

Remember to get Torque Bow or Boomshot as a second weapon. I prefer Torque but Boomshot is a great option too.

When you fight the Snatcher, if he took one of the dummy accounts (or your account), you have time to rescue that player. Just keep shooting with explosives or active rounds.

I didn't have problems with Wakaatu, but I found the Boomshot less reliable than Torque Bow, so remember to find a Torque before fighting Wakaatu.

I don't remember more tips or difficult parts. Obvioulsy getting this achievement playing alone is slower than playing with others, but it's doable.