I like this a lot! I always thought it was silly that you turn a fighter into a one-fight-per-day creature like the wizard once you get hardiness. Also, by the time you got 5 levels of hardiness, you have all-day levels of resistance cheeze anyway, so it might as well be passive so you don't forget about them.
+5 damage is really strong, but since you can only take it once, I'm all down for it!
Whirlwind issues listed by Polytope are legit. Whirlwind was one warrior ability I never minded having a activation for. Oh, what about making it an ability that you can only use once per turn? Then you can just keep the Whirlwind is -4 to hit, and GWW changes that to normal to-hit levels.
Greater Deathblow seems too strong. 5% chance to kill Irenicus is VERY funny, but seems too much. Maybe 5% chance to deal an extra 50 physical damage and 50 magic damage? The magic damage is there in case the guy you hit has stoneskin up. Or, make the attack hit as tho no protections are in place? 5% chance to ignore absolute mantle? Heck yeah! Makes me think the Hulk saying "Puny Mage" after slapping them around the battlefield!
Smite should have a save to negate. Stun is basically death, and if you crit on 25% of your attacks, and deathblow on 5%, laws or probability just means you're gonna wreck it! Maybe the stun can have a penalty equal to your strength damage bonus?