My hardest boss fight was in Metal Gear Solid 2. It was the second to last boss fight, on extreme difficulty (highest you can go), without using any rations (a variation I came up with to make it harder) where you destroy the Metal Gears. I think I ended up having to take out 35-40 of these suckers, and I couldn't get hit by them cause I had so little life one hit would kill me. Took me about 5 days to beat that boss.