Well, there are side bosses on different games that are harder than the actual final boss.
Let's see.
The Weapons in Final Fantasy VII
Omega Weapon in FF VIII
The black summons in FFX
The valyrian woman in Star Ocean ends of time
Perhaps Arhu is a bit of an easter egg in the game. Once you beat him, there is a sense of achievement because he is that hard. There might be an actual achievement in steam but I haven't played the game that much. I haven't even finished it lol.
