In order to successfully unlock the achievement, you will need to find every monsters in the game, and reveal all their weakness. Needless to say the Analyse spell from the Scholar job will help you a great deal in that task.

Here are things you do NOT have to do in order to unlock the achievement:

• Weakness from Challenge and Provoke. This includes "Ancient One" monster from H'aanit chapter two as I did not reveal every weakness from that monsters and I still got the achievement in the end. And as a side note if you reveal some weakness from Ancient One and you run from battle, when you re-engage against it, all weaknesses you previously uncovered are lost. So this was a nice indication for me the game wasn't recording it for this particular battle and thus wouldn't be required to do.
  
  
• All One-Time encounters are not required, this include every boss and all the unique fight where you have only one shot against the enemy. Obviously everything behind the Gate of Finis is not required. Do note there are 10 mini bosses in the game you'll need to reveal their weakness as those enemies actually respawn as normal monsters once you defeat them and it doesn't matter if you reveal weakness on the first fight or subsequent fight...
  
  
• Regarding the 4 missable enemies, the first 2 in Tier 1 area, Meep and Cactus Roller, you won't need them for the achievements as they have been tested as not required. However the other 2 enemies, Buccaneer 1 and Sea Birdian 4, they are required. If you want to secure the achievement, make sure you get those 2.

* Note: Be sure you get all enemies on 1 savefile as trying to get them on multiple savefile won't work. Also if you die for any reason while uncovering some weakness, it is important to understand you have to uncover back what you've lost since last time you saved, in most cases you'll want to reload your auto-save file since this is saving every time you zone in/out and may be easier to remember what you've lost. (Also be mindful that teleporting doesn't auto-save your game).

If it's not your first playthrough of the game you are probably already familiar with the Danger Level System. When zoning to a new area, the Danger Level will be displayed to you giving you an approximation of what level you should be if you want to fight in there.

Regarding this guide the important thing to know is some areas scale in levels by completing a chapter or two. One way the game increase the difficulty of those zone is by adding new stronger monsters to those area while removing some other weaker ones. So this is very important to understand because while most of the monsters leaving an area can be caught elsewhere, this is not always the case. That means, there are monsters you can miss forever if you do not get them beforehand.

Now which area does increase and how?

As a general rule, in Tier 1 area all the zones starts at Danger Level 1 except for the optional dungeons and as you acquire new characters and your party size increase the level of those areas also increase from 1 to 4, 7 and finally 11. For every character you get all the zones in tier 1 gets push by 1 step. There are exception to this like in the Cobblestone area where the danger level doesn't increase at the same rate, but eventually it reach level 11 like in all other places. I don't know why Cobblestone area works differently but the game is full of exceptions so you'll have to live with it. Also if you complete a chapter 2 before having a full party, this will also increase the zones to level 11 as if you had a full party even if you are still soloing.

In Tier 2 areas there are also progression but this time only the roads that leads to other town. All optional dungeon and chapter zones have fixed danger level. Most of those Tier 2 area does increase in level for every 2 chapters' 2 done. In tier 2 area, levels of those zones are all specific, they don't start at the same level and they don't increase at the same pace like in Tier 1 area, specific numbers will be given in the listing. Once again there is an exception, in Wellspring, since there's no chapter 2 quest there, only chapter 3, the Tier 2 area of the Sunlands increase in level upon completing several chapter 3. Upon completing a chapter 4, this will also make the zone increase in level (by 2 steps if I remember right).

None of the Tier 3 area scale in levels. :)

Seriously all those changes in level were a pain to test out and required a lot of time. So enjoy the results of these findings!

Now that you understand how the zone scaling works, or at least you have a general idea of it, let's talk about what you need to do in order to secure your achievement and you push in the right direction.

1) Tier 1 has 2 missable monsters. Those are:

• Meep from the Flatlands and
  
• Cactus Roller from the Sunlands.

If you want to encounter those 2 monsters you need to get them before having a full party of 4 characters, you can still get them with a party of 3 characters. You also need to refrain yourself from completing any chapter 2 as this will also bump the danger level at 11 where those monsters disappear.
* Again, Meep and Cactus Roller have been tested by many people as not required for the achievement, you do however need to get the other 2 missable to get your achievement.

2) Tier 2 has another 2 missable enemies. Those are:

• Sea Birdian 4 from the Coastlands and
  
• Buccanner 1 also from the Coastlands.

You'll be able to get those 2 monsters for as long as Monstruck Coast and West Goldshore Coast are still at level 31 or below. When the level of theses zones hit 34, then it's too late if you missed them. Just get those 2 before you finish too many chapter 2 story and everything will be good.

A bug has been discovered where the weaknesses you uncover are not saved at the end of battle if that enemy gets replaced by another one. This involves the "Proliferate" ability from monsters such as Shambling Weed, Rampant Weed and Mutant Mushroom.

For example, in "The Spectrewood" area you can encounter a group of:
- 2x Creeping Treant + 2x Shambling Weed
If you reveal some weaknesses on Creeping Treant and you first kill both of them and afterward the remaining Shambling Weed use their "Proliferate" ability, and both spot occupied by Creeping Treant gets filled up with other Shambling Weed, then those weaknesses you got during this battle won't save. This bug can also happens if you use Bewildering Grace and an enemy gets transformed. So be careful if you're doing this achievement and this happens to you.

If you ever find yourself in a position where you checked marked everything and you didn't unlock the achievement, you'll have to double check the listing. Everyone who did double check eventually got their achievement. Just make sure you have the missable enemies and you should be good (Meep & Cactus Roller have been tested as not required for the achievement, but Sea Birdian 4 and Buccaneer 1 are required).

First of all I have to give a special thanks to Kezo.
We both went through the same pain of building up that table and at the end we merged our results. We were both missing things but together we did it!

Another special thanks to Icirian, for formatting the listing in a nice way with checkboxes, his addition to the checklist will help you sort out what's left for you to get as checking a box will highlight everywhere else the monster appears. The listing is now sorted between each "Lands" (ie: Flatlands, Frostlands etc.) and there is still the "Alphabetical order" list that still holds everything from all Lands if you need a quick look at what is missing. Since you won't have the rights to edit the original document, you just need to copy it for yourself and you'll be able to work with it easy (you can do that with "File" -> "Make a copy"). Be mindfull that Bosses (the ones you can fight only once) are not required even though they appear in the document, same with "Special enemies".

https://docs.google.com/spreadsheets/d/10bkWrxQQ6xrMQTw8eJxcp-7KtNFykhbRBc-D2rCA1jA/edit?usp=sharing

Good luck to everyone!