Play Kill Confirmed and run around and collect tags. As far as I know KC gives you the most XP per game. Look for challenges. If you have the Elite app installed you can look for the nearest completed challenges and finish them, Also look for weapon challenges to complete.
work through all the perks (again - XP for perk challenges)
engineer is a v. useful perk if you have black hat as your equipment you can earn a lot of XP "hacking" enemy equipment
also, if you have a launcher as your secondary, you can be the "annoying guy shooting down everything"
firstly this role is very useful for your team!
secondly - keeping the skys clear is worth more XP for shooting everythin down