Leveling Traits
The cliche answer would be to say that you should use a Greatsword and a Rifle. Then go down the +Power / +Precision / +Crit Damage route to make yourself a glass cannon dps build. You’d take the Major trait from the critical line which grants +40 Precision per Signet that isn’t currently on a cool down.
My issue is that this “leveling” spec often becomes people’s full time spec. They want to run around the later areas of the game with this same spec, which is often very self-focused and typically party in terms of group support.
What spec should you use to level? I’d honestly say that you should level in whatever spec you plan on playing in the level cap range. By learning your spec as you level, it gives you time to make mental revisions and to test out new changes on various types of mobs throughout the many zones.