Skilling Backstabbing

[Mark11]

Hi, actually i'm lvl 14 and want to get my backstabbing to lvl, means 75 and i have 14 now.
My question is, how can i raise it at best ? Actually i run around at newbie yard in qeynos and backstab the lvl-1 mobs there, no gain so far. I have the impression that the few gains in got in blackburrow were faster, maybe because the mobs there are near my lvl, does lvl of mob make a difference at all ?

Glad for every hint i can get.