Beat the tutorial.
Saw that there was no guide for this game and I thought "Hey, why not?" I've never made one of these before so forgive the rough instructions. This will likely be the first achievement you receive in the game. Just start a playthrough and select "Play Intro" when prompted. Be sure to complete the whole thing and you should receive the achievement after the battle against the Dragon Princess. You will not receive this achievement if you skip the intro, though you may skip it on all subsequent playthroughs after completing it once.
Reach 10 Affinity Points with any Princess.
I feel like it needs to be established that affinity is a stat related to the player's relationship with a certain Reign, not the princess that rules it. Affinity is for the faction, relationship points are for princesses. That being said, you can increase affinity by giving certain gifts to the princess of a given Reign. You can also increase it by completing quests affiliated with that Reign, or aiding the Reign in field battles when they are at war. This achievement basically requires you to max out your affinity with any Reign, so just keep doing stuff to affiliate yourself with them more and you're sure to get it.
This achievement requires you to complete the tutorial without taking any damage. This means you must start a new game and select "play intro", then make your way through the Kobold mines and defeat the first Dragon Princess without getting hit once. It may take you a few attempts but careful movement and patience will see you through it. You really only have to fight the Dragon Princess if you just run by the bats and there is a save point just outside her arena, so the retry time for failed attempts is very forgiving.
Get 10 Progeny.
Same deal as when you needed 1, just like... 10 times more. One thing I can say is that the shifty merchant that visits the Knights Guild INC. at night sells an item that speeds up pregnancies to help hasten your progress. Check out the first progeny achievement for more help.
Beat Dusk Princess using only Slime Princess.
Discover one of the secrets in "The Flow"
Slay a Chiseled Crab.
Upgrade the kobold's mining equipment.
Resurrect a skeleton using the Death Not.
Slay an Hippopotus.
The Hippopotus is a powerful monster known in-game as the "king of the jungle". It basically looks like a hippopotamus covered in leaves to nobody's surprise. It can be found in the jungle in the southern region of the Kingdom. To fight it you just need to find it and select "I'm for the Hippopotus" when prompted. It has two moves, a quick bite and a rather annoying tactic where it starts shaking all around, this move makes it impossible to attack the beast and if you touch it while its happening you'll get hit, hard. It has quite a bit of health, but as long as you fight carefully it shouldn't take for than a few minutes to end it even with early-game gear, its damage is no joke though so be sure to save prior to fighting.
Catch a fairy.
Who doesn't love a good Legend of Zelda reference amiright? To earn this achievement you must first make your way to the Fairy Grove, its a forest of abnormally colored trees behind a mountain range north of the Golem Reign. After arriving you may notice that the numerous fairies around are running away from you when you get close. You'll have to chase one down while mashing the interact button to catch them. Once you do you'll be presented some dialogue options, when this happens, select "Bottle the Fairy" and you'll have your achievement.
Get 50 Progeny.
Same deal as when you needed 1, but I mean... 50 times more? Boy were getting up there now.
Amass 20000 Swirlies.
Swirlies are little candies that function as the Kingdom's currency, and as your way of gaining experience to level up. They can be found in chests around the world, received as rewards for completing quests, or given as a prize for winning a competition like the Holstaur Cow Runs. The most common way to get them, however, is to slay monsters. All of these methods are viable so just use them at your leisure to get 20000 total.
Amass 50000 Swirlies.
Yup, another money gathering achievement, quite a bit more than the 20000 one though. It'll take quite a bit more time but there's really nothing you can do except grind the good grind.
Steal a pair of panties.
Ooooh, scandalous~ This achievement requires you to sneak into a princess's room at night (or during the day depending on the princess' sleep schedule) to steal a pair of her panties. To do so you'll need to approach their castle at the appropriate time and slip by the guard on duty to enter. You can either use a party member skill to sneak by (moss princess and slime princess have the one you need) or use a sleep drug to knock them out. Sleep drugs and other assorted unpleasantness can be purchased from Trevor, the shady merchant who hangs around Faun's Tavern at night. Once inside the castle you'll need to avoid being spotted by the patrolling guards and reach the princess's chambers. There are certain items that make this easier but you can still do it by sheer sneakiness. Once inside the princess's room you just need to interact with the panties on her dresser and choose to steal them. Panty raid complete!
Obtain a dragon egg.
This achievement is pretty straight forward, you just need to find yourself a legendary creature's unattended unborn offspring to steal and raise as your own. It sounds tough but in all honesty this achievement is cake. There are a few dragon eggs in the game, for the easiest one you need to wait until the Desert Reign holds one of its moon festivals. You can tell when this is happening by the incense burners and lamps visible on buildings in the reign from the world map on certain nights. Simply enter the reign and head to the festival plaza, then approach the desert raider looking guy toward the top right of the area. He'll say that he's found something out in the desert and hopes to sell it to someone for 2000 swirlies. Buy it and you'll quickly discover that what you've purchased is in fact a genuine dragon egg and, by extension, a spiffy achievement to match.
Reach the end of the Huge Treasure Dungeon.
Reach the end of the Sphinx' Fortress.
Reach the end of the Sunken Temple.
Defeat Lechef using the pitfall trap in his room.
Reach the Kingdom Below.
Have a Princess reach her maximum weight.
Recruit Amelie.
Recruit Crusader Princess.
Recruit Crystal Princess.
Recruit Dokuro.
Recruit Mimic Princess.
Recruit Moss Princess.
Recruit Scarecrow Princess.
Have Frog "Magic" transform one of your Progeny.
Save a Princess during an enemy siege.
Reach 5 Relationship Points with any Princess.
Obtain a prismatic egg.
Buy a slave at a goblin auction.
Sneak into a Princess' bedroom and have sex with her.
Beat Dragon Princess III.
Defend a city from an enemy siege.
Reach the Kingdom Above.
The Kingdom Above, basically Heaven if you haven't much time, is a peaceful paradise accessible only to the most pure. To the east of the Harvest Reign is a massive tower stretching into the clouds, this is the elevator that takes visitors to the Kingdom Above. Simply enter and be assessed as pure and you'll be transported there. Purity is nothing to worry about at the start of a game, so head over as soon as possible if you really want the achievement. Assuming you are corrupted somehow you'll need to cleanse yourself at the chapel near the Human Reign before entering.
Reach the southern half of the Kingdom.
The northern and southern halves of the Kingdom are divided by a massive mountain range. In the middle of the mountains is the Mouse Reign. In order to reach the southern region, you must make your way through the mineshaft north of the Mouse Reign and enter their capital "Clocktown". After arriving exit the city through the southern gate and descend the mountain, when you emerge from the path you will be in the southern region and receive the achievement.
Break 50 stones.
Just smash some rocks, easy as it gets. First you need to get yourself a blunt weapon like a club or mace, as these are the only kinds that can smash rocks. Then you just need to find a place with plenty of rocks to smash, like the Kobold mines or Mouse Reign mineshaft. Just attacking a rock will smash it, do this 50 times and you get the achievement.
Win a field battle.
Throughout the game, a Reign may decide to declare war on another. When this happens, you may sometimes come across battles between the two factions on the world map, typically somewhere between the two warring Reigns. When this happens, you can approach and are offered the choice of joining the battle to aid either side. All you must do for the achievement is accept to help either side and win the battle.
Get 1 Progeny.
This really involves no crazy effort on your part. Once you're sure that one of your companions is pregnant, just wait the required time for the pregnancy to end. This will be indicated by an egg with an exclamation point appearing above her head. Simply deliver the baby and you have a progeny and an achievement to boot. There are also several unique progeny characters that you can have join your party when you finish their side quests, which will be explained in their own achievements. Alternatively just finding an egg someplace and hatching it will also work, but where's the love, ya know?
Get a Princess pregnant.
Need I explain this concept to anybody? When a man and a woman love each other very much, they have sex, and depending on the fertility of the woman involved a child may be conceived. This is all it'll take for you to receive an achievement and the wonder of eventual childbirth.
Obtain an egg of any race.
This'll happen one of two ways, either deliver an egg after a successful pregnancy, or find one sitting around in the world somewhere. For instance, an egg can be found in the storehouse of the Harvest Reign.
Same deal as when you needed 1, but seriously like... 100 times more?! Damn that's a lot.
This achievement requires you to reach the max rank, 10, with the Knights Guild INC. As you probably know, you increase your rank by completing quests. So, logic would dictate, all you need to do to get the achievement is do a LOT of quests, high rank ones preferably. You'll get it eventually just going about the grind but, just as a tip, you should probably set your days pretty short if your going for this one, as you can only complete one quest per in-game day.
This achievement requires you to smash 500 rocks. If you really just abhor the site of anything igneous, sedimentary or metamorphic then this should be no problem for you. Just use the method found in the 50 rocks achievement but, ya know, way more.
Slay a King Cucco.
The King Cucco is a massive chicken monster that lives in a secluded forest known as the Canopy. Its located up north of the destroyed Kobold Reign, near a place that looks like an opening in the trees. The monster has two moves, a jumping move it does when your above or below it, and a faster peck attack that it uses when your to the left or right of it, that being said the fight is way easier if you just bait the slower move by always being above or below.
Enter a Princess Route.
Unlock a Princess' THICC form.
Reach the maximum level with the Knight.
Reach the maximum level with a Princess.
Complete a quest for the "Knights INC."
Not sure if I really need to guide anybody on this one but just incase... After completing the intro or starting the game having skipped it, you will spawn outside of the Knights INC. guild, this is where you receive quests. Simply enter the guild base and speak to the clerk at the desk to the far right of the tavern. At the start of the game you are at guild level 0, allowing you only two (very easy) available quests. Just accept and complete either of them when prompted by the clerk.
Have sex with an NPC.
It really speaks for itself, doesn't it? This will likely happen innumerable times throughout a playthrough of this game, but if you're going for speed then there are two options I find especially simple. First is the chapel outside the human kingdom just north of the Knights INC. guild, there are several nuns there who may offer to have sex with you should you speak to them, answer "sexy time" when prompted and you'll have your achievement. Even easier is the Knights INC. guild itself, there are typically some NPCs in there who are ready and willing to have sex with you should you ask. A few examples are a dog girl sitting at one of the tables and a goblin at the bottom left of the tavern where merchant princess sets up shop. Neither of these are givens though, as the NPC spawns are seemingly random and change by the day.