I did that, I even put double defense and attack buff, along with haste buff from Felix. It was impossible without a little tweak on the save. I understand the boss fight should be strong, but not so different from the other ones. If you want a powerful boss like that, then at least the minor fights should be stronger as well. Otherwise it will be a save trap, you can clear the entire house except the boss fight. It doesn't give you the option to go away to get stronger as well for the fight. I tried like 30 possible ways (or even more, no kidding) and it was impossible. And even when tweaking the save file I only did a bit and still had to edit more (it wasn't enough). I had previous quests where I wasn't prepared and barely could reach the boss, but didn't have quests on previous quests where the difference between "normal" and "final" fights was so big.