STORY OF SEASONS: Friends of Mineral Town

The basic concept outlined by punkshep in their guide for reaching BF30 applies all the way to BF255 but there is some extra info that might be handy.

It's worth waiting until you have given the Harvest Goddess 100 gifts (and after the start of year 3, as discussed below). At this point she will unlock a 'mine elevator' which makes this achievement much easier. Once it's unlocked, the elevator gives you the option to start from one or more preset floors, depending on how deep you've gone.

Waiting this long should also mean that you have found some power berries, so your maximum stamina will be increased.

The preset floors are 50, 100, 150, 200 and 225, but you will need to manually reach the relevant floor before you can use the elevator to get there. Eg. if you go to BF85, the next time you enter the mine you can travel to BF50, get to 108 and you can get the elevator to BF100, and so on.

What this means is that you don't have to make an epic trip all the way to 255. You can break it up into shorter expeditions where the maximum you'll need to cover is 50 floors, which is not too bad.

Curry Udon is a pretty good recipe to restore stamina, specifically 60 per meal. The ingredients are bought at the store. (Save-scum on the horse races and use the medals to buy dresses, then sell them to Huang for huge profit; when I filled my inventory I made over $2,600,000, but you can only sell to Huang one at a time which is annoying - regardless, money shouldn't be a problem!)

The process, as outlined in the solution mentioned above, is to make a save when you start on a floor. Hunt around and find the ladder or holes*, then reload your save and head straight to the spot - once you load into a floor the design is set, so as long as you have saved after you reached the floor, everything will be the same.

*When you fall through a hole, you will lose stamina. But, you can also fall up to 5 floors. If you have ample food to replenish your stamina, this is the fastest way to make your descent. Not all floors have holes though.

You can potentially alternate between 2 saves but I didn't really need to, with one exception. As mentioned, the design of the floor is set when you reach it. If the floor has something you want (Mythic Ore, for example), you can save on the floor above before descending. Save again when you reach the relevant floor and search for your item. If it's not there, reload the first save and descend again; the design will have changed. If it is there, you can reload the save you made when you reached the floor and head straight for the item, same as above.

I chose the Spring Mine, specifically because there is a recipe on BF255 which you'll need for the food-related achievements (Ketchup, found by digging).

If you leave this until after the start of year 3, you can also find the Travel Stone. From memory I broke some rocks to find it Confirmed it is found in a rock on BF255 and it is very useful. If it is carried in your toolbelt, you can open the town map and use it to teleport anywhere! (Even to the Lake Mine in non-winter months).

There are, of course, goodies to be found in the Lake Mine at BF255 too, and you can see the full list of resources in both mines (with credit to the awesome authors), here:
https://fogu.com/sos3/activities/spring_mine.html
https://fogu.com/sos3/activities/lake_mine.html

So, like I said, this achievement wasn't the tedious slog that I thought it would be. In fact, if you have luck with the holes, you can cover each set of 50 floors very quickly. Best of luck!