Started up a toon last night. Barbarian, of course. I found the Death's Bargain legendary which I never really paid too much mind to but for leveling I just stacked regen so my aura was ticking for ~20k/s. War Cry + Invigorate and I found out Inspiring Presence's 3% HP/s counted as life regen as well so VIT also counted as HP/s effectively for the build. So while running bounties I'd just walk to the bounty location and passively I'd kill everything in my wake without actually having to stop. Not sure it was exactly the most efficient (I had to tune down to normal difficulty but with the way the item worked I'd almost always get the 1.5x XP bonus) but its easy. At least until you can craft a lvl70 with -reqs ... which requires some death's breaths to level up the smith.