I'm currently leveling Mage Guild* and Siphoning trees with Resto staff (Cycle of Life damage buff). Just got to L30 Siphon, L5 Mage, L26 Resto. Need L30 for +10% passive.
Crippling Grasp II, Swallow Soul III, Killer's Blade III, Degeneration II*, Inner Light II*
Morphed skills take a huge amount of XP to level up.
2 DoTs plus 2 direct spell damage. 2 HoT + a 3rd via Execute. (Assassin passive gives MP back too)
Grasp roots for a few seconds, which is nice. I use Soul Tether for AoE dps