My solution is a combination of two strategies listed by Cheese Touch and Jimbo LaFleur but expanded upon to make this achievement, and Lifeline in general a cake walk.
At the beginning of the game head straight to the base and rescue it BUT DO NOT rescue the first survivor, Dr. Thomas Horn.
You still have two companions, no hordes, no sieges, and get full access to your base. You also don't save, so keep that in mind but it shouldn't be an issue.
Then take the next several hours and loot the entire map, saving the section with the first survivor until last. Your character will get tired but just drink energy drinks or coffee and use trucker pills to counter it. There should be enough available on the map to not require the medical lab to manufacture potent stims. Once the entire map is clear you will have approximately 8 grenade launchers in total, along with full food, materials, and a high level of fuel and medicine. The police station and delivery trucks on the highway for me were the most common areas to find grenade launchers, but I am sure there is a bit of randomness to it.
Anyway, once the map is swept clean and your base fully built and stocked, outposts built on your favorite support sites ect, go and finally rescue the first guy. The first siege is the only one you actually have to try on but it is only one wave so its easy.
After the first siege, most everyone in your base becomes playable, so now you can finally give everyone a grenade launcher. Because AI characters have infinite ammo with no friendly fire, there's no worrying about ammo. The AI seems just as effective with the 6 barrel and the single shot grenade launchers because of how they reload after every shot regardless.
Finally give Hawkes a break after being up for 8 game days straight on trucker pills and pick another character to level up.
Since everything has been salvaged prior, you now have ample time to rescue civilians and do other side missions. Once you rescue two or three soldiers (8-10 population) the game stops those missions and since nobody should ever die, that's one less thing to worry about.
If you want to make it even more unfair, wedge two vehicles across the breaks in the walls around your base, the places with the yellow poles, so zombies have more to get stuck on while they wait to explode.
Ammunition as a base resource will be extremely limited so simply call in ammo supply drops so you can keep refilling the mines around your base between sieges. You should get enough influence from the missions between sieges to get 1 - 2 supply drops each round. You can also do the long range drop but the guns that come with it aren't really worth twice the influence cost, and you'll just wind up over stocked with other resources.
Eventually the sieges might require you to do something, so I keep Hawkes at base as my "siege specialist". She is loaded up with ammo, grenades, molotovs, a scoped rifle, ect. When it gets to threat level two, I return to base as whatever character I was using, grab their grenade launcher and switch to my specialist.
Finally, as stated in previous solutions, do not do the final mission, just let the game keep running so sieges occur until you get to your tenth siege and achievement.
Any questions or comments please leave below or feel free to message me. Thx