This is one of my last multiplayer games on PS3, and I feel the same as you. I've done some mind-numbing grinds before, but something about MP3 just puts me off after an hour. I'm doing my best to prioritize level 50 so I don't regret having it incomplete if the servers decide to go down without notice.
One thing I'll mention (which is probably in every guide, but I was too stupid to look) is you must be signed into PSN (and the R* servers) for XP to count towards your profile. I spent about 15 hours doing it offline thinking I only had to play one multiplayer match after I was max level to pop the trophy only to realize none of it counted.