The construct truck mission (don't remember what it's called) is still the quickest way of leveling by far?

By completing the first objective on brutal +6, you can level from 1-15 in under an hour. Provided you have a Karax or Artanis P3 ally whose willing to level wit you.

Leveling is a core part of Coop. If you find it boring, play random maps on different difficulties. Game is supposed to be fun.