I have a new player in my first campaign as a GM, a homebrew that's been running for about fifteen sessions. The rest of the party's versatile and interesting, but my new player "Bob" went with a specialized build that's giving me trouble.
At 9th level with one mythic tier, through some extreme min/maxing (audited as legit), Bob does an 8d6+54 greatsword vital strike at +19, with a crit range of 17-20 doing 16d6 + 108 (again, this is due to mythic). But despite being the party tank, he has fewer than 100 hit points and a so-so AC, which leaves me in a bind. Any enemies capable of surviving the damage of Bob plus other party members for more than two rounds can usually retaliate with enough damage to flat-out kill Bob in return. (Bob has Die Hard, so he's on his feet swinging till death.) Bob hits like he's level 15, but takes punches like he's level 7; as he put it, he could kill a duplicate of himself in one hit without even needing a critical.
I realize I can throw in some super-specialized enemies he can't hit, complicated environments, etc. (this is what I did for his first session). But in the long-term, is there a sensible way to arrange good fights for a party who can dish out massive damage but can't take it? Or should I just ask Bob to scrap this guy and design a more balanced PC? We're all good friends, so there won't be hard feelings around the table no matter the outcome.