Easy experience:
Set missions for Tiny/Hard. The missions will have enemies at one level higher, but you get more experience points.
Get a team together and be social. Team up with someone at a higher level (the higher the better). Use Tiny/Hard missions.
Take your time. Take an extra minute to scout the entire level. Missions are timed; the longer you take, the more experience points you get.
Shogo reference:
In the international district, there is a club named “Club Shogo”. Shogo is an earlier Monolith game, a first person shooter, that was made before they were bought by Warner Brothers.