If anyone ever find one, you can say it here. I didn't find any savegame folder so this must be hidden in the registry somewhere.

Save file is located at C:/User/(user name)/AppData (hidden folder)/Roaming/MMFApplications named as simply "save". Use notepad to open it, navigate down to [globales] change boss=0 to boss=1 then save the file. This works as an alternate method if anyone didn't want to beat hard mode to unlock it regualrly