I found this solution on reddit:
https://www.reddit.com/r/fo4/comments/3wyn21/psa_benevolent_...The author,
https://www.reddit.com/user/lunamoonraker, gave me permission to post this solution here.
If this helps you, go there and give lunamoonraker some love, because this simplifies an arduous task.
I'll post the full details, but here are some summary items.
Requirements:
1 x Settler (moved from other settlement)
1 x Sleeping Bag (or any bed, must be under cover)
1 x Machinegun Turret (5 Defense)
1 x Bar* (20 Bonus Happiness)
Local Leader Rank 2 (min. level 14) No other perk required.
That's it. No super huge amount of settlers, no need of caps in excess of 30k. Basically just level 14.
It will be slow the first day, but then you'll notice progression. It should occur like this:
80>84>88>91>93>95>97>98>99>99>100, I used a Level 3 (Restaurant) to get those numbers, but it can be done with level 2.
Here is the text of the method (and the end of my actual contributions). I did this just last night at Starlight Drive-In, and I can confirm that it works.
The Basic MethodYou can of course add more than these basics but be aware that the more resources you have the more likely an attack which is just distracting really for the purposes of this method. The Settler is Protected so will never die from hostile attacks (only by the player or their companion) therefore is not really a concern for possible attacks.
What you will need;
1 x Settler (moved from other settlement)
1 x Sleeping Bag (or any bed, must be under cover)
1 x Machine-gun Turret (5 Defense)
1 x Bar* (20 Bonus Happiness)
Local Leader Rank 2 (min. level 14) No other perk required.
Note; * Depending on the game localization it can also be called Shop but basically the Food & Drink Rank 2 store.
Supply line is OK but not necessary. No crops planted. No water pumps constructed etc. All Water & Food required for the 1 Settler placed in the Workshop storage, sufficient for around 10-15 in-game days (1 Food/ 1 Water per day) The Settler is assigned from another settlement. You do need the ability to use the Workshop.
I strongly suggest using the Red Rocket Truck Stop settlement. (see reasons in General Notes below) But only if it has not been used already. if so, use another smaller settlement or clear Red Rocket back to no resources or settlers.
Method Go to the settlement and craft a Sleeping Bag (3 Cloth) and place it under cover (i.e. indoors/ under the canopy or you get a penalty)
Craft the basic Machinegun Turret (8 Steel, 1 Circuitry, 2 Gear, 2 Oil) and place it on the roof (out of reach from attack helps)
Build a Stores > Food and Drink > Bar (Rank 2)* (5 Wood, 3 Steel, 500 Caps,)
Place sufficient Purified Water and Food (i.e. Mutfruit) in the Workshop storage. Allow for one of each per day for the Settler.Around 10 in-game days minimum. Better allow for 15-20 of each to be safe (if it runs out Happiness will drop) Remember Mutfruit = 1 Food. All other crops = 0.5 Food so double amounts if using these.
For the achievement the Size bar must move from green to orange colour which signifies a large settlement. The simplest way to do this if you have not done any scraping here is to spam craft 91 x Wooden Crate (3 Wood, 273 total. Can scrap after) Otherwise just craft these until Size turns orange.
Before proceeding confirm via the Workshop interface that all values are 0 (Green not Red) except Defense should be 5, Beds 1, Happiness 50 (base value) and the Size bar is orange. See this screenshot.
Once confirmed Move an existing Settler to the prepared settlement from another settlement via the Workshop interface to the prepared settlement and assign to the Bar store. If all went as planned you now have a settlement already with Happiness 80 which appears to be the maximum level with no Bonus Happiness. See this screenshot.
The process has now started. The Happiness should have reached the maximum 80 without the bonuses and there should be the green Up Arrow next to Happiness indicating it will increase.
Progressing from Happiness 80 > 100 for the AchievementNow we have the pretty tedious part which is unavoidable because the rate at which Happiness will rise is governed, at least partly, by time played and not just in-game time. So you can't simply Sleep for 10 days and have the Settlement reach 100 in a matter of minutes.
After checking various reports of how to do this I found the most reliable way (never 100% sire but seems the better way) was a mix of waiting a bit each in-game day and also Waiting and Sleeping to progress faster than simply leaving the game running (works but some possible pitfalls, see below)
Suggested Daily (In-game) RoutineOnce the settler is assigned I used the following daily (in-game) routine;
First day it wont rise so sit in a chair and Wait until 8:00PM then:
8:00PM > 8:00AMSleep (make another Sleeping Bag for this is easiest. or just Wait)
8:00AM > 12:00AMJust open the Workshop window and idle in-game and watch for the Happiness to tick upwards. Often it will happen literally within 1-2 actual minutes,if so then move to next stage. If not wait for around 10-15 real minutes then move to next stage regardless if it ticked up or not.
12:00 > 8:00PMUse a chair and Wait, then repeat cycle and sleep until the next in-game day.
Repeat until Happiness 100 and the achievement pops up. Around 10 in-game days
Congratulations!
This is the most difficult achievement and only a very small percentage of players get it when checking Steam for example. Not made any easier by the way it currently works.
Hopefully this method at least removes some of the uncertainties and provides a reliable and fairly straight forward way to gain it.
General Notes & Further Details
Which SettlementI strongly suggest the Red Rocket Truck Stop as the settlement to use for this exercise.
It has a small footprint so requires less building to reach the large (orange bar) stage required for the achievement. It is close to Sanctuary Hill for the Settler (not companion. I have not checked if that works yet) and any materials you may still need. Also, it is available without any quests etc. Just use the Workshop.
If you have used it already then the Happiness may be altered, so best to remove all added items (crops, pumps etc.
A bit more on the Waiting Stage
I tested this three times fully with varying numbers of settlers (yes, I am a masochist!) and it hit Happiness 100 around Day 10.
Some times you can wait 30-60 minutes real time and it just doesn't want to tick up for some reason. Probably just the scripts. I found just keeping to the routine if this happened then the next day it would tick soon after waking as normal. Not sure why this happens. Maybe some variable in the script? or just happens.
I also tried saving, reloading and even restarting the game just to see if it helped but there was not regular pattern and it's mostly just a case of grinding the time out and ensuring the green Up Arrow is still there.
Because it only takes 5-10 real time minutes to do a daily cycle, for around 10 in-game days it still takes time but is faster than simply waiting I found.
it's quite possible that simply Waiting rather than Sleeping works also but it just made the time pass a little quicker to roleplay at least the sleeping...