A Farewell to Arms

A word of warning, some of the things required for this meta are among the most RNG-reliant achievements ever implemented in WoW.
Others require specific World Quests to be active and are tied to their completion, so in most cases you only have one try per quest to even attempt the achievement (Bless the Rains Down in Freehold, Revenge is Best Served Speedily, It's Really Getting Out of Hand, A Most Efficient Apocalypse, Adept Sandfisher, Scavenge like a Vulpera, Puzzle Performer, Nazjatarget Eliminated).
Lastly, some require long, time-gated grinds (as well as collections of rare mobs), and may take up to a few months to complete (Undersea Usurper, Mecha-Done, Expedition Leader, Maximum Effort).

To get a head start, focus on Nazjatar achieves, especially Aqua Team Murder Force and Periodic Destruction. They take around 13 weeks to complete, but can be achieved much faster if you're using multiple characters to fulfill different criteria. For Mechagon, just do all the dailies every day for Outside Influences, and gather as many parts as you can for Junkyard Machinist and Junkyard Apprentice.
Islands are on a rotation of 3 specific ones for each week, so getting Island Conqueror will take at least 4 weeks, depending how many you already have, and which ones are available.
Keep an eye out for specific BfA table missions too, it might take a while for them to appear: Azeroth at War: The Barrens, Azeroth at War: After Lordaeron, Azeroth at War: Kalimdor on Fire.
Lastly, don't forget to clear Arathi Warfront for Tour of War, the bosses don't change for a given week so you'll have to wait a long time to get all three.

Dishonorable mentions:

Combating the Corruption: requires killing rare versions of ordinary mobs, which can appear randomly after killing mobs of that type (Only 10 of them are required).
Tell Me A Tale: based purely on RNG, it requires specific mob types to appear on the island for a chance of a drop (Only 10 of them are required).
The Reining Champion: one of the most clunky, unfair, and frustrating achievements in the game, harder that all Darmoon races combined.
Notorious: PvP islands still cannot be entered solo, and require full teams of 3 players on each side.

EDIT: A recent PTR build suggests that many achievements from this meta will be nerfed, and some no longer required.
It's still a very difficult and time-consuming achievement series, but at least it's a little more bearable now.

I love the fact that they are going back and adding these. For casual solo players who like collecting, achievement hunting etc these are evergreen content that you can work on over time, great side content between patches. The fact that some of the achievements are hard is kinda the point.

This company is trying to drive us all insane