Voici la réponse du Support, elle marche !
"Hello,
Unfortunately it appears the achievement may not have been properly recorded.
This typically happens because you are not connected to Steam friends at the time you received the achievement, either due to intentionally setting yourself to "Offline," or due to network connectivity issues.
If you are set to offline in your Steam friends list, please ensure you are set to one of the online statuses, then re-attempt the achievement.
Otherwise, please complete the following steps, then re-attempt the achievement:
Please exit Steam and go to the folder called C:\Program Files\Steam\ (this is the default location for a Steam installation - if you set a different installation directory, you will need to browse to it).
Delete all of the contents of this folder except:
The Steamapps folder and Steam.exe (If 'exe' is not visible, look for the black and white Steam icon)
Then, you will need to launch Steam.exe from within the Steam installation folder, and not from a pre-existing shortcut.
With Steam running, re-test the original issue.
Note: This process will not affect your currently installed games."
En français et plus rapidement : enlevez tous le contenu du dossier Steam sauf le dossier SteamApps et l'application Steam. Lancez l'application pour obtenir de nouveau Steam en entier, lancer Magicka et cliquez sur une magick pour avoir le succès !
