After trying many ways this is the fastest way i can find... to farm marine xp replay mission 11 Home on Ultimate Badass over and over. This will net you 2850 xp in under 5 mins each time. You can do this solo or in co-op, each way is different
Solo - Only Queen spawns
Co-op - Queen and Soldiers spawn
co-op is faster. Looking in the airlock direction where the queen spawns one guy stands inside the far right container and will be keeping the queen busy while the other runs around pressing the switches. Once the 4th switch has been done both of you run over to the 5th and final switch and shoot the queen drawing her over to the loader, then flick that switch to end mission. Just watch out for the alien soldiers that keep spawning.
If the queen does not come over to the loader, then one of you will have to go over to her and lure her onto it.
Rank 60 is 234000 xp total
I strongly recommend you do all the challenges and complete campaign on Ultimate Badass before farming for xp, as by the time you have done all that you should be well into the rank 40's
(I did 1 co-op playthro on Ultimate Badass, then completed all challenges which left me at rank 48)
Edit - Just an idea on how long it takes using this solution, it took me just over a hour and half to go from rank 48 - 60