Need help on tales of vesperia secret mission 3 achievement

[Dark_Rulez89]

Hopefully you guys can help me out on this....For some reason i didn't get the secret mission 3 achievement while fighting Gattuso

I switched the diffuculty to easy mode..Was that the problem?..That i switched to easy mode while fighting the boss?.. the Reason i didn't get the achievement?...Any help will be helpful thankx!!