1. On every level focus on finding the map first. Without it you're blind. Warning: sometimes maps are hidden in secret areas.
   
2. Try to fully clear every level (up to 22 included) rescuing all prisoners (marked by axe on the map), finding all secrets and gold you can find - I guarantee you will miss something, but we'll fix that later.
   
3. All secrets are visible on the map, so look at it frequently to see if there's a room in a spot which looks all covered by walls.
   
4. To break new paths you need:
   
   • Bogdan to push some pillars. Unlocked very early - on the 2nd level.
     
   • Bogdan having his standard weapon to break walls from the side (those can also be destroyed by Janosik's gun or axe).
     
   • Bogdan having bombs to break from the top. Can be bought from a shady dealer met in the village (hub), right from the witch's hut.
     
   • Harpoon from the fishermen shop to break walls underwater.
     
   • Hugo having lightning ability to break lightning barriers. Unlocked quite late, level 20 if I recall.
5. Fishermen, fire runes, gun schematics and spellbooks give upgrades. Use them as soon as you find them.
   
6. Buy all upgrades and items offered by any NPC. If (when) you find all treasures you will have 17k spare gold in your pockets.
   
7. Skip long swimming parts untill you get water breathing.
   
8. Last animal to interact is the dog in Strongman's house, accessible after level 20.
   
9. Last fisherman, hidden in underground lake on level 22 (bottom right side) will give you gold compass. You will have all gold marked on the map. When you obtain that return to the hub and check every completed level, starting from #1 and all sub-levels for any missed gold. Remember about the secret level accessed by swimming left in the hub and the big doors with a crystal mark.
   
10. After entering level 23 go straight to the right, save the game and interact with the sheep to end the game - this will unlock the bad ending, since you didn't kill the end boss.
    
11. Load last save, fully clear levels 23 and 24, but don't rescue any prisoners on level 23. This should give you the Jackpot achievement. After killing Batory get back to the sheep and end the game again - this will unlock the good ending.
    
12. Load again, free all prisoners on level 23 and end the game again - this will unlock the very good ending.

Keep all that in mind and you should have another game added to your completionist showcase in less than 20 hours :)

Chain Reaction 2 is the worst level from the game. The only one when you have to be very fast and cannot make any mistakes. You need to start the detonations and immediately run along the breaking path. You can try collecting all gold, but you can also leave that for a second run (the breakables reset after re-entering the level) when you have all the pillars already pushed by Bogdan. I recommend not doing this level when you discover it, but wait until Janka has wall climbing ability. It helps a lot.