Well I've heard thorn sucks but at the same time watched some one go beast with it and say it's amazing so that's personal preference as far as how to do it if your not a hunter use a void class your special and grenades do void damage that way you can do void damage while you still have your primary weapon also try and find a better void weapon my personal recommendation is high impact and range those will give the highest damage per shot at the greatest distance also try playing something like skirmish smaller maps with more corners works well with the fusion rifle other than that keep grinding and you'll get there eventually good luck