I try to keep things simple, though feel free to let me know if you want more hints!
Siren’s Son: Meet Sebastian with Will lower than 25 in Ch 2, and then the MC will be bewitched.
Leave No Stone Unturned: Visit all the following 9 places (no need to follow the order) in Ch 2: The Mighty, Main Square, Tavern, Temple, Blacksmith’s, Bookshop, Observatory, Beth House, and Mayor’s House, and then you will get the achievement at the end of Ch 2.
Whodunnit: (It’s been a while since I played this game, so I am not really 100% sure, but I can confirm it later) Go to the Mayor’s House, check every floor from then ground to the top, and then select the correct answer to 5 questions about the culprit and their motive. Do this before going to the Observatory (the final showdown).
Dear Diary: Find all the 7 pieces of diary (2 in Ch 2, 1 in Ch 3, 1 in Ch 4, 1 in Ch 6 and finally 2 in Ch 8), and then you will get the achievement at the end of Ch 8.