BIG Thanks to TonySki (First Comment)
i can confirm that it is true what he wrote because i just got the achievement finally.
You have to get all achievements in the same savegame.
i THOUGHT i have done that, but i remembered that after the online achievements (which i did first), i chose the wrong difficulty and you can not change it without resetting savegame.
i know this will be no solution, but i have 24/25 achievements unlocked and this achievement is at 91% and refuse to unlock.
i already sent the developer of this game Teyon an email.
If someone would also message them on social media about this issue (i have no accounts there) it would be nice:
and if you also want to send them an email, all the info you need is here:https://teyon.com/company/contact/