This will ONLY work while the game server is running and your friends leaderboard is showing!

Due to the fact that the save data for the game is stored in different files it is possible to trigger a manual steam achievement unlock by replaying the mission with an empty achievements file. This guide shows you a way to get such one and then keep playing your normal profile.

This works for achievements that are unlocked in missions!
I am unsure if grinding achievements can be fixed that way too.

I used Total Commander to easier handle stuff but of course this can all be done with any File Manager like Windows Explorer.

The difference is: You will need the proper files for the level and grinding stats prior to the achievement you need to unlock.
The Method is simple:
Replace the coresponding files for Level and Grind information with a clean one and grind up to the achieve you need or use one that is close to the achieve you need, like a lvl 19 one thats short to ding 20 for the achievement.
Because i will NOT offer any files to this guide i will also not explain that method in more detail!

Locate your game profile inside your Windows User directory.
Where Autositz is your windows user name!


Make a copy of your COMPLETE profile folder. The number displayed is your SteamID64.
If you have more then one profile numbers in there. Make sure you use the correct profile. Sites like SteamRep[www.steamrep.com] can show your ID64 if you use a custom url.

• Start the game and go into the options menu.
  
• Reset your profile and accept the question with YES that you are sure.
  
• Exit the game

Go to your COPY folder with your old profile data and
select all files EXCEPT "[ID64]Achieves.save"
to copy them into your normal profile folder.


You can see the selected files that need to be copied on the LEFT side and the destination folder on the RIGHT side. Overwrite the destination files with your old profile data except the Achievements.

• Start the game and select to use LOCAL profile on the question window that pops up.
  
  
• Start a map and it should show the achievements as NOT unlocked now.
  
  
• After playing the map and unlocking the achievement via gameplay the achievement should also trigger for Steam too.
  
  
• You can also try to take the other way as lined out in the last section of this guide.

• Go to the game profile folder c:\Users\Autositz\AppData\Roaming\Prime World Defenders\ and delete your profile.
  
• Rename the COPY of your profile by deleting " - Copy" from the folder name.
  
• Start the game and AGAIN select to use LOCAL profile when the question pops up.

You now have your original profile with your stats and achievements back in play.

As Ren pointed out in the comments below, it seems to be possible to unlock achievements by going through the steps as I lined them out up to the point where you launch a mission.
From that point you take a detour by not launching the mission but going into achievements to confirm that all are locked.
Close the game afterwards and also copy the achievements file you have not copied earlier back from your BACKUP into your ACTUAL profile.
Launch the game again and go into achievements, the missing steam achievements should pop now.

I have not tested nor can I confirm this method but it seems to have worked for Ren.