Coral Island

Definitely one of the longer possible to unlock achievements but you're here for a long time not a good time on Coral Island eh? This should unlock the night you go to bed after banking enough experience to level your last skill up to 10 at the day recap screen. First off, for all skills you're not crazy, it only totals the xp at the end of the day which can make it hard to track in real time or even devise a strategy to know what you're doing. Secondly, the higher tier/value the item you gain xp from, the more xp you get i.e. Osmium products are worth more xp than silver. I shall list the categories you need to max out with any relevant efficient ways I found to grind them (spoiler there's not a lot of great time saves). Also the elixirs sold in the Community Center do not add any XP to help you reach mastery, they just give you the missing skill point you wouldn't get from finishing mastery. No need to fret over those.

Farming:
do anything directly related to farming. Easy you'll spend lots of your days here. Use the hoe, water stuff, harvest crops (faster with a scythe). Level up your tools and you can eventually hold X to use them on larger swathes of ground which helps. I gave up on watering things around year 2 and relied on sprinklers for that so i could pursue other masteries.

Ranching:
Pick up ranch loot, pet your guys, feed sick animals medicine, I don't think putting hay in the bins counts so you can leave a lot of grass out nearby your barns and coops to easy feed your critters. As you try to fill out the "ship every animal product" cheev you'll gain more and more affection for your animals and they'll drop better and better quality junk. Follow my advice to just build a ton of cows and then you can get rid of them once you level ranching up all the way.

Foraging:
Both CUTTING DOWN TREES and picking up random land and sea bits (clams, chestnuts and things). I would recommend once you get the upper left area of the map past the mines unlocked, start actively farming hardwood trees on your farm. You get the most xp from cutting down entire hardwood standing full grown mahogany and poplar trees. You should have super drops by now and get scads of the seeds, but make sure you space the trees appropriately or they will not fully grow. If you get bored you can always plant pine and maples as long as they grow past a pile of seeds you can get SOME xp. It doesn't hurt to pick up forage junk either if you have the extra inventory and sell bin space free.

Mining:
Break rocks. The nicer the rock (geodes and osmium) give the best xp. You will spend a LOT of your time in the mine any way so don't stress on this one. Do make sure you upgrade and enchant your pickaxe asap to get Hit Drop, Rare Drop, and Extra Drop. (Pro tip once you get the enchantment to purple grade thats as far as they go ditch any extra fossils and whatnot). Good time to do this in winter because in your early years there won't be much farm work.

Diving:
Clear trash and hit kelp. You'll need lots of kelp and trash is pretty useful too. You'll get about 6 or 7 levels in this just finishing the mermaid bs quest line, and if you need more there's always focus farming kelp daily, or just grinding out trash in one of the undersea cave dungeons. You will need lots of trash strikes anyway to find all of the artifacts and fossils that are in the underwater loot table. Same as mining osmium is worth more so don't sleep on swinging on any kelp piles you see in your travels, energy withstanding.

Catching:
Bugs and undersea critters. You'll just want the net out most of the time until you catch everything in the lists for the museum and offerings. The hardest to find critters just take a little bit of patience and an osmium net. You need to get in front of them with the net poised and let them come to you. Otherwise once you get all the perks and an Os net, most creepy crawlies are a breeze to catch. This one felt like less of a chore than foraging.

Fishing:
Catch fish. You do catch a fair amount of trash and you can selectively cut the line when the trash appears (there will be no fight squiggles or movement from the float) More expensive the better but you'll catch loads of fish in your time especially leveling up to osmium and getting the rarer fish for the museum. I rarely if ever used bait and would just fish on the most expensive potential node of the day according to the computer's fish report. Get the computer and the upgrades it helps for this and bugs/critters. You could also check out the wiki there's a full list there as well.

Combat:
The last one I got because even after clearing ALL the mine quests, all the mining mastery, all the fossils, gems, offerings, ANYTHING to do with having to be in the mines for something else I only got this to 6. After a lot of trial and error it looks like just spending your day grinding all 4 mines (do fire, wind, then the others) level 30-39 is the best gain of xp. Mob drops are basically worthless but can be used for crafting, but you will end up gaining a lot of mining xp and mats from that in your search for the way down to the next floor. Sorry I have no great tips on this score but you'll be familiar with the mines by the time you get to this. Take the damage perks and combat medic asap. I ended up using the Flameguard greatsword which you can buy from the adventurers guild, or get for free I think in one of the festival games. You can only increase your damage not your defense or hp. (When Grinding JUST for xp) kill everything on floor then move on to the rng of finding the next floor, which is often but not consistently near the largest grouping of mineral nodes.
Be comforted that this achievement isn't broken at least and you will never have to go in the mines again.

Updated: A lot of players have outlined roughly that farming reproducing slimes using a combination of scent lures and spring traps is the way to go, but I cannot attest to this myself because I will not be going in the mines again. If a brave soul can give me a detailed explanation of their reproduction steps I will happily add it as a preferred combat (non gathering/other xp) alternative!