So if you completed the game (or any other reason) and can't restart it cause the portal just is a loop to the start of the level, for fixing it you'll have to delete your save of the game, but I didn't find it in the steam files and any other place in my pc but I find a way to fix it, here's how:

1. Open the game
   
2. Press F12 (steam screenshot, it will open a window of codes and stuff)
   
3. Go to the 'Application' tab
   
4. In the left side will have some weird things but look for Storage> IndexedDB> localforage> keyvaluepairs
   
5. Rightclick on keyvaluepairs and select "Clear"
   
6. Restart the game :)

This is what the screen look like

So, a lot of players are experiencing problem in getting the boss achievements, that occurs because (this game is broken) the achievements aren't for the boss you kill, but for a counter you receive for killing them. Weird right? Let me explain:
A lot of people might have killed the first boss and died the same time and WOA surprise, achievement for boss 1, then that person kills the boss 1 again and WTF?! achievement for boss 2, or the same for boss 2, or 3, you can get all boss achievements in any boss, the only problem is, you cannot close the game, the counter resets.
I killed boss 1, got my achievement normal, killed the boss 2 got nothing, why? Maybe I closed the game, I don't remember... Then when I killed the boss 3 I got the achievement for the boss 2, so no boss 3 achievement.

The best way if you have the bug problem is to:

• Reset game
  
• Get yourself to Flying Eye #1stBoss (you can use the bug later on this guide to get there faster)
  
• Kill him the same time you die so you get a counter (1st boss achiev)
  
• Kill him the same time you die so you get the second counter (2nd boss achiev)
  
• Kil him one more time so you get the Boss-3 achievement (dying or not, doesn't matter)

Advices:
You need to hit him 6 times, so just lose your HP and when you're going to hit the 6th time, jump on bullets or on him.(its boring, annoying, you will need to do this some times to master it)
You don't need to worry with deaths, you don't lose your counters, just don't close the game or kill him without dying.
When you successfuly kill him the same time you die the screen will change color (at least, with my experience was that what happened)

If by accident you killed him and didn't die, IMMEDIATELY PRESS ESC, cause you will pass level, and we don't want that. Be careful, you will lose your counters and need to redo everything so, it's better to die a lot than to kill the boss without dying.

For this part of the guide a special thanks for the discussions in the community and the players who did manage to discover this!

When I was trying to get my bugged achievements I recognize that if you have a saved game and try to start a new one, when you enter the portal (not the bugged one plz) you will skip 1 level.
Exemple: You have a save in like level 23, you reopen the game and go for new game, you do the 1st level and when you enter the portal, you will be thrown to level 24.

So basicly you just need to redo level 1 10 times to get in the Flying Eye

1. Open the game
   
2. Have a save, whatever the level it is. Like, complete level 1, saved level 2.
   
3. Close the game
   
4. Open the game
   
5. Choose to start a new game
   
6. Redo level 1, get to next level, in this case, level 3.
   
7. Close the game
   
8. Keep doing this till you get to the level you want.

I did this so I could get to Flying Eye faster and try to get my 3rd Boss achievement.

Good luck y'all

And Dev, please fix this game. It's a nice game to pass time but so many bugs...