Mages like Mage, although inherently fragile, but if you can't penetrate their magic shield, don't expect to chip away even a bit of their health.
It is precisely because of the sturdiness of the magic shield that Mages have time to chant and cast spells, launching ultimate attacks that require a long time to charge.
Lin Xun couldn't directly defeat the Great Sage, not because he didn't have enough defense or health, but purely because he couldn't break through the "Knowledge Light Wall," that turtle shell.
The Great Sage's Shell itself is of Mythical+ Level, and so should be his signature Skills.
As long as he could break through the Mythical+ Level defense magic "Knowledge Light Wall," with his current attack damage, he could easily insta-kill the fragile Mage.
[...]
[You have possessed the Shell of 'Mage of the Direct Vision Branch Association seeking Knowledge'!]