Assassination is the easiest to level at your level and above, really. Once you get SnD rolling, just 2-3pt Rupture the mob and 5pt Envenom. By that time it should be dead. The Rupture will refill your energy and you will have no down time since Envenom refreshes SnD. If you want to, you can get the Deadly Momentum glyph as well and keep Recuperate up.

Combat is just really energy starved for leveling speed. The main builder in Sub requires you to be behind the target. If you have to spam Hemo, you aren't going to do much damage.