This game contains a lot of micro management phases followed by a lot of sequences of idling and farming. Be aware that this is not your standard idling game!

When not running, you won't make any progress in the game. BUT! The game has a concept of "bonus" time, based on a percentage of your offline time, i.e. the time the game does not run. It starts at 0% and can grow up till 50%. You get a 5% upgrade when finding (interacting with) one of the hidden elves.
This bonus time is great for farming skills, route and speeches with characters.

Stats:


Some areas of the game are expensive to explore, the flooded/muddy grass in the south west and the south east of the starting point (before the Firefly queen and the Necromancer). To travel more easily without spending too much energy, you can buy the Sturdy Boots in the town at the Ye old Shoemaker (thanks @Codeki for the reminder).

You can unlock several skills in the game by completing some actions.


Some remarks:

Rodent affinity: Great for early game. Be careful to not pick up 20 rodents or else you'll get the rodent cape with the corresponding buff.
Critter and beasts affinities: Great for mid-late game.
Villager affinity: Easy to miss! Great for mid-late game.
Spirit control: Spirit infused creatures are fire bats and the Firefly queen. I don't know if this skill is a game changer, but more mana is always good I guess.
Body control and hand to hand: More damage is always good.
Defense efficiency and parrying: Just update both. The game has a complex defense mechanism explained here if you are interested:

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2679598685

You will notice that the game allows you to farm and improve your skills as much as you want. So you may wonder: when should I stop ?

My answer would be: just stop when you reached your objectives (e.g. finishing the story, getting the achievements). As "reference", here are my stats after getting all the achievements of this guide, minus the five last ones:

You can unlock several buffs in the game by speaking to the characters.


Some remarks:
The buff effects are displayed at their max level.
All buffs' max levels are 5, except "Max health" which is 6.

Rodent cape: As soon as you get 20 dead rats in your inventory, you automatically get a cape with armor 5. It replaces your existing armor (shield). If you get another armor (shield), it replaces the cape.
If you want to sell rats or throw them in the bonfire, be careful to have max 19 of them, or sell/burn them in two batches.
Way of the guard: It may not look like it, but this is easily the best buff if the game. With it, you can race through the map killing critters after critters and filling a never-ending 20k mana pool, ignoring fireflies altogether. This buff is the main reason of putting points in the spirit stat.
Friend of the fireflies: The sword of mana replaces your current weapon, and is replaced by any weapon you pick up. It goes up to 60% attack and I assume it cannot be forged at the Blacksmith (never tried it).
I see dead people: The ghost provide each 1 free point you can choose to distribute in the stat of your choice. There is one special ghost in the right part of the city that is required to save the chancellor. See underneath in the Story Achievements.
The Chancellor's Gold: The gold caches are marked by a red cross on the ground and contain each 10 gold pieces,

Free at last
Escape the loop

Note: After breaking all the 4 crystals in the same loop once, they will always be broken, allowing to get the other story achievements more easily.

• Kill the Caporal and get his blood.
  
• Kill the Samurai and get his blood.
  
• Kill the Necromancer and get his blood.
  
• Kill the Firefly queen and get his blood.
  
• Go to the mountain top break all four crystals
  
• Put the slug in the bonfire

Obviously some farming is needed for that. As soon as you have a loop that can reach the four crystals, you can let the loop run and you will ultimately reach the goal.

Find love
Get the girl and break the loop

• Get the wine
  
• The four crystals must have been broken(see above)
  
• Talk to the necromancer with the wine in inventory and get the "love" buff
  
• Put the slug in the bonfire

Finish him!
Slay the king and break the loop

First of all, to access the chancellor: Speak to the necromancer to get the skill I see dead people to max level. Go to the right part of the city and wait a bit: you should see a puzzled ghost (it doesn't give stats). Interact with it and you will learn that you can cross the boxes. Now your goal is to cross these boxes as soon as possible to kill the assassin and save the chancellor.

• Save the chancellor
  
• Get the wine
  
• The four crystals must have been broken(see above)
  
• Take the slug and follow the bloody steps in the mountains
  
• Pick the amulet on the King
  
• Use the lever
  
• Talk to the chancellor
  
• Talk to the necromancer with the wine in inventory and get the "love" buff
  
• Open the throne room door
  
• Kill the demon
  
• Put the slug in the bonfire

For the 3 stats achievements, you should have unlocked the mountain top to have enough possibility for XP gain. If stuck, increase the XP rewards of the critters by throwing their remains in the bonfire, the same for the citizens (see above). Otherwise, just farm the route again and again.

I can fight GOD
Get your body level over 100

The easiest of the three. Limit yourself to 9k mana, minimal heart level and then just dump every point in the body stat. Killing everything on the map (including characters) + the ghosts should give you enough points. Also the bonfire(s) and the scarecrows give a bit of XP so include them in your loop.

I am made of mana
Get your spirit level over 100

Not much to say about this one. Limit yourself to 30-40 body, minimal heart level and then just dump every point in the spirit stat. Same strategy: killing everything on the map, the ghosts, the bonfire(s) and the scarecrows.

FWOOOOSH
Get your heart level over 100

Mix of the both hints: Limit yourself to 30-40 body and 9k mana, then just dump every point in the heart stat. Same strategy: killing everything on the map, the ghosts, the bonfire(s) and the scarecrows. This one is fast to test :)

Around the around
Reach 5000 total loops

You should get this naturally. For info, after getting all the achievements of this guide (80% of all achievements), I was at 68 hours game time and 7k loops.

Stuck in a time loop
Reach 10000 total loops

If you didn't get it naturally, just create a mini-loop with 2 steps (one step seems to make the game lag) and run it in autoloop with accelerated time from the hourglass :D

Batman would be proud
Break both scarecrows in a single loop

There is one scarecrow in the starting zone with 10 usages, one in the west zone with 100 usages. Just interact with them 10 (resp. 100) times till they are broken in the same loop.

Nothing personal, kid
Kill the Chancellor before the Assassin

You need to kill the chancellor before the assassin kills him, without killing the assassin first. The chancellor is a bit stronger than the assassin, but this achievement is not too hard to obtain when farming the route.

Chancellor for king !
Die by the Chancellor's hand with the amulet in your inventory

Follow the steps of the Finish him! story achievement. The goal now is, after taking the amulet from the body of the king, to attack the chancellor and die by his hand.
The problem is that is quite weak and you could theoretically lock yourself out of the achievement since every time you vanquish him, it is becoming easier the next time.
A trick to achieve this goal to to attack him while having only half an heart: hence, he will most likely win. To be with half an heart, attack all the creatures on the mountain top. If necessary, use "speak" on an empty case (i.e. you talk to yourself a bit). Since you are regularly losing health on the mountain top, you can fine tune your loop to enter the Underground Steam with half an heart.

A very sharp stick
Upgrade the club 10 times in a single loop

Not too hard. Just collect 110+ gold and upgrade the stick 10 times at the blacksmith.

For instance: dig all the chancellor caches, speak to all the villagers 7 times and pawn their hats, kill the old man and the fish merchant.

For more hints about getting gold, see:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2853876313

Wood is all I need
Kill the demon with the club and wooden shield

This one is quite hard as the demon hit like a truck.
Don't forget to improve your stick 10+ times at the smith (See the other achievement A very sharp stick) and be fully stuffed with health potions.
I unlocked it with Mana=47, Speed=5. Body=83.

Kung-Fu master
Kill a guard with your bare hands

Another hard one. The guards in the city become attackable after you escaped your first loop. Dump the most stat points possible you can in body, avoid interacting with any firefly (since the mana sword granted by Friend of the fireflies will block this achievement). Maybe you can still get it with a shield (I can't remember).
If you are stuck, farm the skills and the route some more.

Heartbreak
Kill the necromancer after dating her

• Get the wine
  
• Talk to the necromancer with the wine in inventory and get the "love" buff
  
• Kill the necromancer :'(

Firefly queen is Best Girl
Talk to the Firefly Queen with the slug in your inventory, without having killed or interacted with any Fireflies/Firebats

This is easier at it may sound: in the late game, with enough in spirit, you can just zoom through the critters, keeping your mana always high thanks to the buff Way of the guard, removing the needs for fireflies and such. Go to the mountain top, take the slug and go to the Firefly queen. You can take a couple of (big) mana potions with you for safety.

Elf catcher
Find all elves

There are 10 elves hidden in the game (one in each map). Capturing an elf grant you 5% of offline time (see above). You just have to interact with each one of them once, after which it disappears.

For each elf location, see:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2690787626

Familiarity overload
Reach crossing level 200 on a square of the map

You will reach it naturally mid-game when one of the squares (most likely in the starting zone) reaches level 200.

Beggar
Talk 7 times to each citizen in town in a single loop

Talk to each citizen (the four guys with hats) 7 times. They give your 1 gold piece each time so you get 28 gold pieces in total from it.

You are the worst guards ever !
Talk to the Caporal with 4 hats in your inventory

Love the pic :) Kill the four citizens and talk to the caporal with the four hats in your inventory.

Who's a good boy ?
Pet the dog

Just interact with the dog right to the necromancer.

A short rest
Interact with the bench

The bench is located in the right part of the city, right before the boxes. Just interact with it.

I can see my house from here!
Use a lookout tower 13 times in one loop

As the description says. There are plenty of towers in the game. You can get this without problem in the mid-game.

Sorry I didn't get these ones so I have no tip for them. Go look on the forum :)

GENOCIDE
Kill everyone and break the loop

Saint looper
Get all 3 endings without murdering people in your final loop

Speedrun
Get all 3 endings with a loop under 20 seconds

Rich
Get all 3 endings with at least 100 gold in inventory

Filthy rich
Get all 3 endings with at least 350 gold in inventory