Estimated achievement difficulty for 1000: 2/10 [Achievement Difficulty Rating]
Offline: 20 
Approximate time to 1000: 4-6 hours [Estimated Time to 100%]
Minimum number of playthroughs needed: 2
Missable achievements: Yes
Does difficulty affect Achievements: Yes
Unobtainable/Glitched Achievements: No
Glitchy Achievements: No
Extra equipment needed?: No
Discover and investigate a forgotten underwater city built in an intriguing Art-Deco style. This former utopia hides many spine-chilling secrets and supernatural forces, the remains of which still lurk in every corner. Face the ancient evil that is hiding in the deepest chambers of the city of Eden. Hold your breath and dive into the game to experience the wonders of the underworld for yourself.
Step 1: Expert Difficulty + HOPs
Start on expert to get it out of the way so your second playthrough is a bit easier. It is not much more difficult when using a guide, which can be found here. During this playthrough, make sure you do not skip any puzzles or hidden object scenes, as there are achievements tied to completing all of both.
Step 2: Bonus Chapter
The bonus chapter will unlock after completing the main adventure, and you simply need to complete it in any way to unlock its achievement, so feel free to use hints and skip anything.
Step 3: Dominos
You will now need to play the game a second time, but instead of completing the HOPs, you will have to use the skip feature and play the domino mini-games instead. You can do this on any difficulty. You can also use hints all over the place and skip all the puzzles to speed this playthrough along. The only thing to try for is completing a domino game in under one minute.
[XBA would like to thank The Pants Party for this Roadmap]