Start as Rogue. Take Lash of Pain in the second magic tree for 2nd level. List goes from there.
3rd-6th lvl: Fury Rogue talents.
7th lvl: Armor passive in Magic Tree
8th lvl-11th lvl: Magic Mage talents up to hp passive. Then max out other Magic side for Polymorph. Then take Pyroblast at level 15.
You can now Polymorph every enemy, adrenaline rush (for high hp enemies), Cold Blood, then Pyroblast. Repeat until the enemy dies.
I ignore Warrior path (Heroic Strike) pretty much period. It has no cool passives, and the abilities aren't as strong against the big enemies as the other trees. Heroic Strike also rolls off of your weapon damage. Meaning some hits are teeny tiny if you roll a 1. And some are strong if you roll higher. Lash of Pain is flat damage. It doesn't roll for the damage, just deals a set amount.
Last thing is to stack agility items for two reasons. Enough agility let's you basically auto crit without having to use Cold Blood, this saving energy. Plus more agility means more energy. Rogue is best because extra agility.
Early game you'll be using a lot of Cold Blood + Lash of Pain, with Frost Nova on the bigger enemies. Late game is Polymorph, Cold Blood + Pyroblast.
Adrenaline Rush is a must.
Hope this helps :)