[Snake]

Yup, we'd like a boss to surpass Metal Gear.

Preference is it being super resistant to Hexers too. (Aka, immune to Poison, super resistant to Dark. Or straight up un-hexable.) Or straight up immune to bullshit tactics such as people who spam knockdown, poison and stuns for easy wins.

Or hey, something like Magus from Chrono Trigger, every round (or every 3-5 rounds) it shifts an elemental 'barrier' after an AoE attack of the type they're becoming weak to (like Lantern Bearer, but global range). So every other element gets resisted/absorbed by them. For this, it could have two barriers at once, one for physical elements (so blunt, slash and pierce) and other for magical elements (all elements available, including akashic and null)

The Physical elements get shuffled every round end, so the boss will always have 25% weakness to one, 75% resistance to other and absorption for another. This one will keep 'mysterious', in order to make it more challenging for non-mages, who are pretty much the majority of characters in existence. (Example:
Round 1 - The boss resists slash, while is weak to blunt, and absorbs pierce.)

The Magical elements will be a little more tricky. The boss performs an AoE as they change their elemental weakness to the element they used in the attack, while resisting every other element, and the 'mirror element' to it becomes an 'absorb'. (Example:
Round 1 - "The Boss unleashes a massive burst of Lightning! - The Boss receives 75% weakness to lightning, 75% resistance to all elements and now absorbs fire damage.
Round 2 - "The Boss unleashes a massive burst of Null! - The Boss receives 75% weakness to null, 75% resistance to all elements and now absorbs akashic.)