ThePiggyman has it.
1. Though I might not look it, I'm a brutal opponent, practically impossible to defeat. - Melville's duel with Guillaume is rigged.
2. I will concede that there exist circumstances in which such a hypothetical defeat would be plausible, even likely. But why risk it? You certainly don't have to take that chance. - Guillaume is totally optional - all of his duels, including the two you can win, can be skipped. If you're terrified of the "risk" that a fat merchant with a pitchfork provides.
3. Pick on someone my own size? Hoho! I think not, squirts! - Guillaume has a feud with Melville and is randomly picking on Shabon because, well, I guess because he's an asshole. "Hoho" is his verbal tic. I was hoping to mislead people into thinking 'giant' here, but no, it's just cruelty to kids.
Your move, ThePiggyman. Sadly I didn't get into hyping his devastating deathblows, with classic quotes like "Deathblow Attack! The most furious of flames!!!" (Gee, I wonder what he's going to do.)