PCGamingWiki[www.pcgamingwiki.com] is the best source for this, and it doesn't disappoint here. Just note to use the Steam option, not the WIndows one:

<Steam-folder>\userdata\<user-id>\1552550\remote\

Where your Steam folder is the install folder for Steam (default in Windows 10 is %ProgramFiles(x86)%\Steam\), and the user ID is a numerical ID assigned to your Steam account (you are, after all, just a number to them).

To get the value of your own steam ID:

• Open Steam, and click your profile name near the top right corner of the screen.
  
• Select "Account Details" from the drop-down menu.
  
• Your steamID is directly under your account name (aka username)

So you can open %ProgramFiles(x86)%\Steam\userdata\ in Windows explorer, select your Steam ID, and then go to the folder 1552550, which is the SteamID for the Castlevania Advance Collection.

From there, the "remote" folder is where all of the save games are stored. data_008_0000.bin seems to be the save files stored in all games from the in-game save feature, while the rest of the files are save states made using the Advance collection's menu:

• Files with data_012_**** are Circle of the Moon
  
• Files with data_014_**** are Harmony of Dissonance
  
• Files with data_012_**** are Aria of Sorrow
  
• Files with data_012_**** are Dracula X

With this you can back-up your save files in another folder, or add new save files, like what I have shared in the next section.

Here are save files made right before getting every achievement in the game.[drive.google.com]

What you need to do is copy the desired files into the "remote" folder as described in the last section. In some games, you need to do this with the game open or the Steam Cloud save back-up feature turned off, but I didn't have any trouble with this game. But if you are having trouble, try turning off the steam cloud back-ups in the game's properties (right-click on the game in your steam library, go to Properties.


Copy the files in your folder, open the desired game, and load the save state. The other error that may occur, is that you may need to make save states first and then copy over the files; I had some issues when trying this on the other Steam account in the family.

Once in the game, the achievements should unlock. In Circle of the Moon and some of Aria or Sorrow, you need to hit the boss once or twice for it to unlock. In Dracula X, you may need to walk through a door first. For the Aria of Sorrow achievement with capturing all of the souls ("data_016_0000" in "Aria of Sorrow - Endings"), you do need to capture the last soul, a shadow knight, but I went into a hex editor and jacked up Soma's stats to make it easy for you.

Feel free to use these as you wish, but I recommend trying to play the games and have fun to get the achievements. but if you missed a boss achievement or are sick of the RNG of soul capturing, these files are here to help.

Hopefully this helps you in either backing up your save files or getting that one last nagging achievement! The files should all unlock an achievement, I tested them on another Steam account in my family (and someday my kids will notice and wonder how they 100%ed this game in 20 minutes!).

Let me know if you need further explanation or have any thoughts in the comments.