Given the fair acceptance and welcoming my guide for getting the Undercover Neurosurgeon achievement has got (linked in the relevant section below), I have finally decided to do a comprehensive guide for all fourteen achievements in this game. I have tried to make it simple and easy to understand. Yet, if you find a way this guide could be improved, feel free to leave a comment below. Thanks in advance.

A piece of advice
Monster Loves You! is a fairly short game by most standards. Even if it has some replay value, chances are you are not putting more than a dozen hours at the very best. It is to some extent countered by the fact that the game is family-friendly, and not only you but younger and older siblings and acquaintances can enjoy it. However, if your main objective is getting all the achievements in the game, I strongly recommend you experimenting by yourself and stop reading this guide. Please make use of this guide as a last resort, when you have tried everything you can come up with. You need to have got Dissolution and Ascension endings at least once each for fully understanding the terminology and happenings discussed in the guide.

An important remark
Pretty much as you would expect in any decent create-your-own-adventure media, there is not only a good amount of endings but also several ways to reach the same ending. Since I assume you are here for the achievements, I will only post the clearest and most straightforward one. In any case, you should go off the beaten track and see what happens. For example, try to workaround a way to avert the Ascension/Dissolution event ;).

An explanation of the structure
The achievements are divided into "Dissolution achievements" and "Ascension achievements". This division should help you with finding the achievement you are looking for easily, since there are seven achievements directly or indirectly related with Dissolution and seven others that require you to Ascend.

Dissolve into Bravery, Cleverness, Ferocity, Honesty or Kindness (Primary Statistics)

• Try to get the best rating possible in the primary statistic you want to dissolve into.
  
• Accept your fate of dissolving into the Spawning Vat. Alternatively, fail the Ascension event.
  
• When retrospectively thinking about your past life, state your wish of being remembered for your words or actions. Select words for Cleverness and Honesty. Select actions for Bravery and Ferocity. Select any of them for Kindness.
  
• Of the three choices you are presented, select the one that relates with the primary statistic you want to dissolve into.
  
• Pass the dice roll*.
  
• Dissolve.
  *Failing the dice roll will result in Dissolve into Mediocrity

Dissolve into Mediocrity

• Try to get the worst rating possible in one primary statistic.
  
• Procceed as you would when dissolving into a primary statistic (see previous subsection). When selecting the feat you want to be remembered for, select what is proper for that primary statistic you were scoring low in.
  
• Fail the dice roll*.
  
• Dissolve.
  *Passing the dice roll will result in Dissolve into the pertinent primary statistic.

Legend of Departure

• When getting the trigger for the Ascension event (just as you exhaust the days of your adulthood), go home to wait for the time the Ascension event happens (as opposed to going directly to the event or doing the fern mission).
  
• When at home, select the option to flee.
  
• Confirm.
  
• Flee.
  Secret: there is at least another method of getting this ending, maybe even two. Failing the other way(s) will result in especial Dissolution events.

All these achievements require you to Ascend. As such, for the matter of following the instructions, it is assumed you have passed the Ascension event successfully and you are already an Elder, unless otherwise stated. All the Ascension achievements/endings trigger after you get your monster's life recap and are about to get the Human-Monster encounter.

Remember that here I am only showing ONE way to obtain each ending. You can move from some "pre-endings" to non-associated endings using Respect options (for example, you can provoke a conflict with humans via Respect options even if by statistics you were about to get a Peace, Combined prosperity ending).

Please note that "bad Relations"/"low Respect" is thought to mean "smaller than 50" and "good Relations"/"high Respect" is thought to mean "greater or equal to 50".

Undercover Neurosurgeon
Please check my guide.

Peace, Combined Prosperity

• Good Relations Humans
  
• Good Relations Monsters
  
• No Respect requirement (you do not have to convince the Monsters of anything at all)

War, All Humans Dead

• Good Relations Humans
  
• Bad Relations Monsters
  
• High Respect
  
• Command Monsters using the corresponding option

War, All Monsters Dead

• Bad Relations Humans
  
• Bad Relations Monsters
  
• Low Respect
  
• Try to help Monsters fight against Humans or tell them to flee but fail due to low Respect

War, Endless Fighting

• Good Relations Humans
  
• Good Relations Monsters
  
• No Respect requirement
  
• Slash a Human in the throat, thus provoking an unintended conflict

Monsters are Pets

• Bad Relations Humans
  
• Good Relations Monsters
  
• Low Respect
  
• Try to warn Monsters about the upcoming Humans but fail due to low Respect

Monster Exodus

• Bad Relations Humans
  
• Bad Relations Monsters
  
• High Respect
  
• Convince Monsters to leave town to live in the mountains, far from Humans

• Carmen, despite being used in-game to name the human town, is a common female first name in Spanish speaking countries. Carmen is also one of the best known operas, composed by Georges Bizet with the libretto being made by the lesser-known Henri Meilhac and Ludovic Halévy (the opera itself is based on a novella by another Frenchman, Prosper Mérimée). Not coincidentally, the opera is set in 19th century Spain, and the main character is a Romani by the name of Carmen.
  
• Clicking your monster will sometimes yield a rough phonetic transcription of the Spanish phrase "¿Hablas español?" (Do you speak Spanish?).