[Autumn]

This has been an issue for a little while now, it's just not as a wide known one, mutating a turtle shell makes it unarmored, and when it's unarmored it stacks with Ogata Waraji and Weathered Body.

To put it into perspective":
Fully upgraded Turtle shell: 10% physical resistance from the front and sides, 20% from the back. (as well as a hefty armor value, hitting 23 with carapace+solid)
Ogata Waraji: 10% Damage Resistance when paired with Unarmored
Weathered body: 10% Damage Resistance when Unarmored and wielding a fist or non-shield in your off-hand.

And then to top it off, all of these tools are available to any martial artist:
Weir Konnen: anywhere from 5-15% Flat Physical DEF
Golden Glow: 10% Physical Magical DR
Ice Point Guard: 10 DEF/RES

It's fine if people want to be physically resistant to all hell and back but what's not fine is that Kip-up negates the penalty of this armor by a fairly large margin for most martial artists, thusly it's become a staple for them to acquire in order to snuff out most physical damage builds with little issue.

My only suggested nerf for this armor in specific is that it disables kip-up, that's all, this will prevent martial artists from being the tankiest class in the game and also getting out of Knockdown which is supposed to be the downside of the armor itself.