Hello there. After playing the game through for three times and unlocking all of the achievements I've noticed most of the them aren't unlocked by more than 20% of the playerbase. And so I thought it would be a good idea to make a short guide with some hints on the achievements. Not a full-fledged walkthrough though, as there are many choices and consequences in the game, and listing all that would be outside of the scope of this guide.

The game has considerable replayability and it implies you'll be making at least a couple playthroughs. So information provided is with this fact in mind. I'll also try to keep it as spoiler-friendly as possible.

For your convenience I have divided the guide into the following sections:

Playthrough Achievements - Case opened, Case in Progress, Case Closed; as well as the endings for the game - The Matriarch, One of the Elite, Clear Consience.
Skill-related Achievements - Might Makes Right, Intelligence is Sexy, Silver Tongued Demon, Jack of All Trades, Scholar, 0451.
Caffeine Achievements - Caffeine Junkie, Decaffeinated.
Missions Achievements - The Inspector, The Analyzer, The Investigator, The Detective, The Tactician, The Diplomat, The Interrogator, The Decoder.
Ladies Achievements - A Partner in Crime, Late Night Horrors Remedy, Lady in Red, Persuasive Argument, Ladies Man, Triple Threat.

Case Opened, Case in Progress, Case Closed - you'll have all of these unlocked by the time you've finished the game, so they are automatic.

The Matriarch, One of the Elite, Clear Conscience - those are hidden achievements depending on your ending. To grab all of them in one playthrough I suggest you save before entering section F in Biozan.

The following set of achievements mostly depends on what stats you level throughout the game and will take several playthroughs to unlock. They are also sort of automatic. Mind that you'll get ~25 points overall depending on your actions and side quests, so maxing two stats while not levelling the third one at all is entirely possible.

Might Makes Right - 10 in Athletics by the game end.
Intelligence is Sexy - 10 in Erudition by the game end.
Silver Tongued Demon - 10 in Charisma by the game end.
Jack of All Trades - 7+ in every stat by the game end.

The next two achievements require an Erudition-focused playthrough.

0451 - when you get to investigate the warehouse in the Industrial District, one where the mercenaries are, you'll have to get a code for the western door. If you have high Erudition (8 is needed for Computer Knowledge book) you'll be able to hack the panel to reveal an iconic code - 0451.
Scholar- awarded for reading all books in the library. For that you'll need a total of 12 Erudition. It doesn't exclusively yeild you this achievement, but also Existential Philosophy will prove very useful in the end.

There are two more achievements, which revolve around the caffeine system, Caffeine-junkie and Decaffeinated. While caffeinated, all points of interest during investigations are highlighted, so I suggest going for this one first, and then, when you've learned all investigations - the second.
It's pretty straightforward ones - make sure to (not)drink any caffeine every day and you're golden. Caffeine comes from soda machines and coffee in the cafe primarly, so if you're going for decaffeinated make sure to pass all Gwen's invitations to have breakfast with her or only drink water instead.

Mission Achievements relates to the achievements you can get during the main quests involving detective work. Some of their descriptions on Steam may appear confusing (like Pelados investigation) so I put them together under the common name like that. I suggest unlocking those while caffeinated, so all points of interest during the investigations are highlighted and you don't miss out on anything.

The Inspector - Full points for Sancras Park investigation. Correct answers are: "was sitting on the bench", "grabbed" and "it happened between midnight and morning".

The Analyzer - Full points for brothers' house investigation. Correct answers are "doctor told them to leave town", "they were medicating". It's the easiest one to get, judging from the achievements stats.

The Investigator - Full points for Dr. Alvarado investigation. Corrects answers are "noone knew where is she", "someone made her dissappear".

The Tactician - Full points for docks shootout. To get it you basically have to successfully pass four action screens with no officer casualties. Make sure you have at least 3 Athletics and 3 Erudition by that point in the game. First screen is 3 erudition check. Second screen is 3 Athletics check. Third screen gives you several options - you need to have 5 in Athletics, 4 in Charisma or 3 in Erudition to nail it - persuade an officer to distract mermen, climb the crane or operate it. Last screen is where you have to shoot the fleeing mermen, don't let them go. If you did everything right, you'll get this achievement, as well as a bonus and police respect.

The Detective - Full points for docks investigation. Remember to check everything, including the archives to discover the origins of the weapons. Correct asnwers are "Important supplies", "No, they didn't want to blow everything" and "Got weapons from a crate nearby".

The Diplomat - This one is a bit trickier than others. It is awarded for full points while dealing with Erwin during the first visit to Pelados. You can get up to 4 points during the visit and achievement requires all four of them:
- comment on knowing about the loyalty task (requires mermen society book read)
- deal with two mermen upstairs non-lethally, otherwise there's penalty of -1 point
- fix the machinery in the room with two mermen (requires 7 Erudition)
- compliment Erwin after returning from the loyalty quest (requires 5 Charisma)

The Interrogator- This one is awarded for a perfect interrogation of Hanzel. Talk to Elyssa in the corridor before starting it, she'll give up the successful tactics - no attempts to make him doubt his morals, only his cause, and no insults or intimidation or anything that can offend him. Ask what was his plan, what did humans do to deserve it, who did he get the information from, say you'd be able to help him if you knew the name, and don't say it's all lies. Elyssa and Solane will then do the job for you and Hanzel gives up the name of Evdokim Kalinin.

The Decoder- Full points on Evdokim house investigation. Correct answers are - "he knew he was going to die soon" and one about the secret message for Miroslava (not "he clearly overestimated you"). It is also necessary to have Gwen in the party or have Existential Philosophy knowledge by the time of investigation, otherwise Hank is unable to decode the message and you won't get full points for that one.

The last, but not the least section is for ladies' achievements.

Partner in Crime - to get Gwen you have to be nice to her, date her and have breakfast with her at every opportunity you get. The most important choices for Gwen are kissing her after the first date and saying you meant it on the next day. It is also important to show how you're willing to sacrifice yourself for her in an encounter with the mercenaries. After that you come to her room and get an option to "talk to her for a minute".

Late Night Horrors Remedy - to get Miroslava you have to always be supportive and compassionate to her, never doubt or question her. When she asks Hank to come to her house alone, if previous encounters were pleasant, she'll invite Hank to the gym with her. Kiss afterwards. Again, show support and understanding of her condition, no matter what. During "Late Night Horrors" talk sweet stuff to her and choose to stay.

Lady in Red - getting Alzyra is a bit trickier than other girls. You'll want to do it on your Charisma/secondary Erudition playthrough, as she requires lots of it if you want to bed her. With high Charisma you'll get an option to compliment her on every interaction, so do that. She actually responds well to Hank telling Solane Miroslava has "hot, smoking body, and nice breasts" xD. With high Erudition and Biochemistry knowledge, you'll be able to impress her on day 2 and also suggest she's a vampire later on. She admits to find intelligence the sexiest thing about men.During both dates, make sure to feed her with the most expensive stuff and treat her. There's also two occassion when she argues with Gwen and Miroslava. If you want to earn your entry into Alzyra's panties make sure to side with her on every occasion. At least that helps. If you persevered you'll have an option to date her for the third time (at her room) before going with her to check on Miroslava's condition.

Triple Threat - you can earn this achievement on Day 7 after Hanzel interrogation. You'll just have to be in relationship with Gwen, Miroslava and Alzyra at the same time. Being intimate with them, however, is not a requirement. Just dating them will do.

Persuasive Argument - you'll have a chance to get this achievement in the late game. To free Elyssa, you'll need high Charisma and Erudition of at least 5 for Merman Society book. To free her, passing Merman Society check and Charisma 5 check is enough. There's also Charisma 8 check, which could supposedly serve as a replacement for Merman Society one.

Ladies Man - for that you'll need to have good reputation with all major females by the game end. Complete all Medina's missions and be supportive to her. Don't be violent on any missions including Elyssa, show her your knowledge of merman society and free her later. For Solane, behave as professionally as possible and don't hide any of your discoveries from her - major spoiler ahead she knows everything from the start anyway. For Gwen, Alzyra and Miroslava - be in relationship with them, dating level at least, and don't get on the bad side of any of them. This means staying neutral on every occasion ladies argue among themselves, as well as very, very, very careful choices during "Triple Threat".

A guarding angel - keep all of your chicks alive. There are several notable checkpoints where some of them may die, but luckally avoiding them is easy enough. Solane - don't side against her, even if for the sake of the achievement. Though there are ways to keep her alive even if you side against her - with good equipment and smoke grenade (needs 12 Athletics). Miroslava - don't let the mercenaries kill her, it is easily avoidable if any of your stats is high enough. These are the most obvious ones. Medina's probably also in potential danger, if you did the quests involving police (docks shootout, Hanzel capture) in particularly bad way - didn't check this one though.