When I took mission 64 I was thinking "let's just see what the most powerful boss in the game can do and I'll come back later" I wasn't even prepared for the fight and I defeated him on my first "test try". That was for laughs, for me the most hard-to-beat boss fight was Attacus, maybe because I was kinda weak at that point of the game but I found it more challenging than Vercingetorix. (only difficult about him is his name )