Hardest Boss?

Of the bosses in the game, who would you say is the hardest? I've been thinking about this for a while.

At first, I thought it was Master Mold. But then I decided it was definitely Shadow King.

The reason being is that I leveled up to level 40 and beat Master Mold using Colossus without losing a single team member on my first try (whereas the other times I beat the game I had been level 35 and got torn to shreds practically ever time)

But Shadow King, I lose several times every single time I fight him and you only have one person to fight him and you can't level up.
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Video games don't kill people. Stupid people kill people.