Disclaimer: This is my first guide so I'm going to try to keep it as simple as I can make it without getting too complex. (I wanted to write a guide explaining how to do this somewhere because I know how much of a pain in the ass this achievement can be.)
From my experience, after playing-through old man a total of 9 times.. I can conclude that in order to get this achievement there are certain guidelines you must adhere to.
1. in order to get this achievement you must complete the whole old man scenario in one sitting. I'm not sure if its a coding bug or what not but if you quit out of the old man scenario and load back into it later it causes the Scenario to go into some kind of weird laggy state. If this happens you'll know because everything will get extremely laggy and when you acquire certain OBJ's such as using spacebar to get intel, grabbing top secret documents, etc. the circle animation thingy will be all screwed up and will go really slow. so the smoother that is, the more likely your game state is still good to go.
2. play on recruit, I don't know if it really helps at all but its what I did and it made my experience of getting the achieve a lot easier on my life.
3.save as least as possible, die as least as possible. I know this is what you want to do anyways but I tried as much as I could to limit how many times the game state would reload so that any cache the game was running off of would not become corrupt. (I was very superstitious about what all I did because I wanted the achievement so badly)
4. drive with land or water vehicles as much as you can. You CAN use Aircraft but only do this sparingly or when you feel that you need to in order to get somewhere or blow something up, etc. The reason I say this is because when you are flying around really fast all of the map is getting loaded into your save and it could corrupt your game state. (use at your own risk)
Onto the guide, so basically what I did was I just rushed all the primary objectives as fast as I could because the longer you spend in the Scenario the more corrupted your save will get so you want to get everything done as fast as possible. do all of the main stuff for Samjo in the beginning like blowing up the house, rescuing the captive, etc. then once you've rescued the captive get the amount of NVG's that Samjo needs(I think its about 5 or what not.) turn them into any stash and then once you have those done immediately do all the beginning keystone objectives. blow up the AA sites if it makes it easier for you idc.
once you get the objective where you need to infiltrate the island where the dome lab is to blow it up, drop everything you're doing and rush to the island. you don't need to worry about doing anything else for time sake. (you'll know where it is if you've played through the scenario long enough.) my easiest method of doing this is to pretty much just skydive in to avoid all the AA and other unnecessary BS. Don't let them see you and take 2 or 3 explosive charges in your Vest with you to blow up the trucks inside the lab when your safe enough distance and get to Keystone as fast as possible, kill him and his squad mates on the beach. Personally I didn't want to move the cure out of the backpack because I didn't want to glitch the Scenario so I just took the whole backpack.
Then once you have the Atrox cure or whatever its called you need to get into a boat or whatever u want and get to the city where the insurgent drop off is. Beware of Gendarmerie and CSAT guarding the city they WILL be patrolling the streets with sniper rifles and they will quickly gang up on you if you let them. during this time while you're running to the drop off Samjo should call you about 2 times, this is a good sign it means that your game state is still running smoothly. run up to the drop off and let Samjo say everything he needs to say. Drop off the cure inside the little explosives box inside the blue van, if everything went right and the Arma 3 Gods blessed you on this day then Samjo should call you right after congratulating you for making the right choice or something like that and you will get an OBJ to leave the city immediately after. do this and try to watch out for more Gendarmerie patrols and don't get killed. Samjo should call you within a few minutes of you trying to leave the city and tells you that everyone wants to kill you and you need to leave Tanoa. At that point its in your hands to get to the edge of the map to trigger the cinematic and Bada-Bing-Bada-boom you got the Devils Due achievement.
I apologies if this was long, like I said my first guide so there you have it. Also I want to give a big shout out to the Arma 3 old man reddit the people that posted in there really helped me to figure out what was going on with the Scenario and how buggy it was.
If you have questions you can message me on steam, good luck out there MEC!