I would recommend to buy dark power first. On 4/4 it gives 15% more dmg for 40 seconds. With my 15% cd its only 9 seconds before I can use it again. My base dmg is 500 with dark power its 575 And remember also shaman and other party members get more dmg ;)
on top i use rage weapon (10% more dmg) 9,9 % chance to activate with rage crystal on armor. Also i use dmg incrrase relic on heal ;) sometimes i make it 700+ dmg if all kicks in. I d say 40-60% of time i have 630+ dmg (500 base dmg)
Next expert skills depends on weather you go for pvp or pve. B