Well first off, when you make a new character you kinda have to go through the motions and just do normal (or hard) missions until you get access to bounties. Once there, just grab as many bounties as you can, wether it be PvE or PvP (or more recently, Queens Wrath). Try looking at bounties that can be done in one mission mode, for example, like "killing a mission boss", and the options are "killing x enemies before they hit you" and "getting x melee hits", you'd want to go for those as you could probably finish all of them in one swoop. Other than that, I just run patrol missions when im really really close to the next level, but the main (and fastest imo ) is bounties. Hope it helps