I'm "normal level strong" and end-game:ish, (I think). I two-shot the boss I think is the final main quest one (there's usually post-game bosses too.) I can barely scratch him. So I assume he is one of those bosses where you have to Final Fantasy - level tweak things perfectly to stand even a chance.

The Water God Dragon is an optional DLC boss from DX with level 100 and has 200.000 HP. So you should bring your ultimate gear and equipment.