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Making an 8th-level boss would be an ideal solution; the damage from the hero's attack can be fixed and made minimal against it. Playing with 4x4 creatures is quite inconvenient, so let it be a boss in the Temple of the Nagas, for example.
Regarding abilities: immunity to water; area-of-effect attack around itself (I think it can be implemented through a pattern attack, and an attack on a 5x5 square); the Ent's roots would be a good addition (like entangling with tentacles, but there's a problem with the visuals); a stunning strike (would be good with a mass attack); infinite retaliation. I would also add 50% resistance to ranged attacks, as such a creature would be very vulnerable to blocking its path with walls and simple ranged attacks (although, depending on the direction of the mod in which it appears, there are options to teach it to destroy such obstacles).
Stats, as mentioned above, should be determined relative to the final set of abilities.