Must say I am iffy about the Improved Mettle thing. It basically means the only viable tactic to kill this guy is hp damage or no-save just die.

I should probably not be iffy about it, however.

Anyway.

Endurance and Improved Toughness as bonus feats at first level.

You should probably remove the "may select another feat" part. I would, in fact, suggest a secondary feat for these cases (Diehard for Endurance and... More Improved Toughness for Improved Toughness?)

In fact. I suggest pulling Improved Toughness as a Bonus feat to the second level. It is so bland in there.