Dark Forest Hills
New Moon (1/8 nights)
It takes, if sleeping exactly on time, ~1h20 to rush to another new moon. Let's just assume you have access to /checkspawns legendary (not everyone does although I think it should default as an accessible permission). This allows you a way to time the legendary spawn so that it will line up with that new moon almost every time.
There is, at max, a 30% chance of a legendary spawning. Let's assume the entire 64 block radius around the player is not only dark forest hills, but cleared out and spawnable space. The game will try to find a block for the legend to spawn x amount of times, and with that many spawnable blocks, the number stays hovering around at that optimal 30%.
But this is where the issue arises; In 1.16, for whatever reason, the sword trio (Cobalion/Terrakion/Virizion) spawn location was changed to all "Hills" biomes. A pretty pointless change, that comes with drastic side effects that were clearly not thought out beforehand. Virizion spawns during the day, so we can forget about him. Terrakion and Cobalion both spawn at night, in Hills (including dark forest hills), with a weight of 3.0. This means, with simple math, Darkrai has approx. 4.76% chance of being the legendary picked between the three. You factor that in with the 30% (at the very most optimal conditions possible) of a legendary attempt happening, and you get a 1.4% chance of getting a Darkrai heading into a New Moon (with a spawn attempt lined up). That, on average, will take around 50 attempts (for a 50% chance additive) Combine this with the expected weight time of 1h20 (keep in mind this is always sleeping, meaning you have to basically always be at your PC to keep this pace up), it will take approx. 66 hours of grinding to get Darkrai.
That's an insane number for one legendary, don't you think? And that's only if you sleep perfectly, have access to checkspawns and time it every single time, and have a perfectly cleared out area. Take away checkspawns (which many server players can't access because of ignorant admins), and it's literally a lost cause entirely. I like the difficulty of Darkrai, but like how it was in 1.12. New Moon, no other legends interfering.
Now I think, with proper strategizes, this is a very possible task, although difficult, with a working Kyogre. Kyogre, as I reported 7 months ago, is supposed to give thunder on rain dance (which you can sleep in, therefore removing the 1h20 mandatory wait with 7 kyogres. And yes, this is 100% worth doing for Cresselia/Darkrai, it saves so much time and if you don't see that then you haven't given this as much thought as I have). However since 1.16, this is bugged and it only gives regular rain. It is supposed to still give thunder, the debug message and a developer confirmed this to me. I used this strategy to catch Darkrai in 1.16 and while it is, on average, difficult, it's very doable with the proper strategy, which is a good thing. I used the weather command when I used Kyogre's Rain Dance as a temporary fix. But most people can't do that to fix the bug. With a working Kyogre, I think the difficulty is in a good place.
-Revert Sword Trio spawns to 1.12 versions.
-Fix Kyogre bug (really should happen either way and I hope it's not being ignored)
-Change the way Darkrai spawns. Possibly some sort of rare NPC quest that's having a nightmare, that then gives you tasks towards getting Darkrai. Idk!
Thanks for reading.