stuck at lvl 12, can't do mars because of the lvl 15 missions, can't do strikes because the last 3 teams i played with have quit. Great game, so fun.
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If pvp isn't your can of beans then bounties are your friend. Turning in 2 or 3 at a time is almost a sure way to level up when your around level 12. Another good way is to do missions on hard, that nets a little more xp. If you can manage pvp even a little, same strategy works here. Grab some pvp bounties and do a few at a time.