I would definitely second Dennis' warning--there are lots of advantages to going with two trees if you are going to run Pit and Countess, and there are Cold Immunes in Durance3, too, if you get a bad map or mis-teleport and wake up Blood Lords. I use Hydra/Orb, but there are lots of combos that will work. There are cold, fire, and lightning immunes in the pit, so pure fire or light would have the same problems. You may not kill as quicky as you would by concentrating on one tree, but to run all of the areas you listed, I would suggest two.