When you get to the last boss of the game (The Devil) he will ask you to give him your soul contracts and to join his team. If you answer "Yes", the game will end and you will get this achievement. (You don't lose the SaveGame)

Same thing as the achievement above, but if you answer "No" to the Devil asking you for your contracts, you'll enter the final boss fight. Kill him and you'll get this achievement.

You just need to find any of the secret shortcuts on the world map. You can find one at the start of the game and once you've reached the other side you'll immediately unlock this achievement.

There are a total of 10 secret coins hidden on the map. Finding all these coins is necessary to get the Butter-and-Egg Man achievement. Some of them are shown below in the images...

Defeat a boss using only the mini-plane bullets, the easiest I've found to do is Hilda Berg in Inkwell Isle I or the Mini Boss in King Dice.

To get this achievement, you must beat the mini-bosses and then fight King Dice without being hit once (or at least finish with 3 lives).

To get this achievement you'll have to complete all the levels without killing an enemy in the platforming levels (Run and Gun). Finishing a level in this way will give you a P rating, which is the highest you can get in Run and Gun levels.

This one is a bit tricky. First you have to buy the broken relic from the store (it costs 1 Coin). A Ghost should appear in the Cemetery if you have the relic, there you can interact with all the tombstones as a code panel. This is where the puzzle begins, go to those competitors (in the second image) who will give you the instructions for your code (each code on the tombstones is randomized for each player). I recommend setting your game to English as it's easier to understand the positions they'll tell you (up, down, left...). You must press in order on the tombstones the answers to the lines from the first to the third competitor or else it won't work and you must always start following the instructions from the middle tombstone! After all this you'll unlock the fight with the secret boss, just kill it and you'll get the achievement.

Once you have completed the above achievement, you have unlocked the power of the relic. You must defeat all the bosses in the DLC with the relic equipped, with it equipped the weapons are random and change when you stop shooting, and the defenses don't charge the meter and you always start with 1 HP, after defeating them all with the relic you will unlock this last achievement.