- Estimated achievement difficulty: 4/10 [Achievement Difficulty Rating]
- Offline: 24 
- Online: 0 
- Approximate amount of time to 1000: ~10 hours for 1 playthrough, ~14 hours if doing 2 [Estimated Time to 100%]
- Minimum number of playthroughs needed: 1
- Missable achievements: 3 - Academy of Umbrellas (100G), Hot Pot (25G), Meal for a King (25G)
- Does difficulty affect achievements: No
- Unobtainable/glitched achievements: None
Time to wake up, little crow. Welcome to Death's Door, a lovely little "soulslike" with simple but engaging gameplay, a beautiful story and well-designed levels that make getting all 24 achievements a complete joy.
Your first step will be deciding if you want to go for the Academy of Umbrellas (100G) achievement on your first run, which will require you to complete the game using only the umbrella as your weapon. This makes the game a bit more difficult as your melee damage will be cut in half (0.5 vs. 1 for the Reaper's sword), but realistically it is nothing that will make you pull your hair out, as the lack of damage can be compensated by either leveling your melee stat or going all in on magic damage. Also, you'll miss out on using some of the cool weapons you get, but it's your choice! See the Academy of Umbrellas (100G) description for more information.
Assuming you want to knock everything out in a single playthrough you'll want to do nothing before sprinting to grab the umbrella - don't even swing your sword in the hub which some people have said will negate the achievement - and then complete everything the game has to offer. There are two missable achievements other than Academy of Umbrellas (100G): Hot Pot (25G) & Meal for a King (25G) which take place during two boss fights you'll encounter as you play. Everything else can be mopped up after the game is completed, so either follow the video guides referenced in the individual achievement descriptions as you play through or explore freely and mop up at the end.
Playthrough 2 (If Desired):
If you don't want to limit yourself to the umbrella on your first playthrough, it's fairly quick to speed-run the game with the umbrella, only worrying about key story elements and collectables you find along the way. The game isn't that difficult to start with, and once you understand where to go and how to solve puzzles the second playthrough can be done in 3 or 4 hours.
If you missed any shiny things, seeds, pots, or shrines there are a plethora of video guides to help you find them which are linked below and in the respective achievements. Some are fairly well hidden, so don't feel bad utilizing the resources at your disposal. Notably, the game provides some help when mopping up by highlighting doors with a red outline if there are still objectives within, and finding the shiny thing "Rusty Garden Trowel" will help you narrow down seeds you may have missed while Pothead will guide you towards pots that are missing seeds at the end of the game.
All-in-all, Death's Door has quite a few collectable-related achievements, but fortunately the gameplay is enjoyable enough and the world beautiful enough that exploring every nook and cranny is more enjoyable than a slog.