***** FIXED. ITS TAKEN 3 YEARS BUT THIS ACHIEVEMENT HAS NOW BEEN FIXED BY THE DEVELOPERS. If you already have the all the other achievements just log in to the game load it up and it will unlock. if like me you stopped playing because of the broken achievment, its time to revisit a great game and get unlocking those achievments*************
No tracked gamers have this achievement, this is officially listed as an unobtainable achievement
In theory this achievement is supposed to unlock once all the other 49 achievements have been earned. Some people have claimed that reinstalling the game then re-entering the last boss is a fix, however I believe these to be trolls/lies, as trueachievements would have picked up on at least one person earning this.
Further research on my part has revealed the developers are aware of this bug but as the games sales figures were a major disappointment they are in no rush, but hopefully a patch will at some point be released to fix this.