Easily beat hardcore at level 10 with this setup:
My warrior had everything in the dodge tree except throw. Also all the passives in the 2H tree and the last four abilities. Last he had "Victory is Ours" from leadership.
My mage got every skill in and earth and general magic trees. The leftover point were put into dodge, where I got salto and most of the other skills.
My priest started by filling up the chaos magic tree, getting chaos shield as soon as possible and only putting enough points in to unlock the last skill. Then I went into the holy tree, also only putting enough points to unlock the last skill. The most important skills here are smite, group heal and holy weapons. This will make the demon area extremely easy. Put all extra points into the general magic tree.
 "Villainous"){kind=icon}
 "Hexologic"){kind=icon}
 "Wondrous Lands"){kind=icon}
 "kill 'em all"){kind=icon}
 "Lightbot 2.0"){kind=icon}
Woooo! I'm preeeetty sure the hard badge isn't impossible-badge-level, but it's definitely tough.