To reach Level 30 (current max) as quickly as possible, complete all story missions on the Normal difficulty. You do not get any extra XP for completing story missions on the Hard difficulty, and barely get any additional XP from killing enemies on the Hard difficulty. Make sure to buy the "Experienced Agent" perk immediately after completing Story Mission 5, where you rescue Captain Benitez and activate the Security wing, to get a 10% XP gain from all activities. Complete Encounters that give a good amount of XP. You normally cannot replay an Encounter after completing it, but if you join a game of another player that has not yet completed the Encounter, you can replay it and get the same amount of XP for completing it again. To easily level up a low level character, especially a second character, play with at least one other Level 30 player and have them complete Story missions, Encounters, and Side missions while you stay back and away from enemies to quickly reach Level 30.