Reaching level 50 through natural progression, i.e. doing literally everything in the game and then some by supplementing it with quests from DLCs could take forever. For the uninitiated the best and most rewarding method for capping the level cap would be by attaining a baseline 100 percent completion by doing all the quests and side quests. My preferred boosting method is somewhere late game by farming both IwaJira or either version of the Vault Sentinel such as the Empyrean Sentinel or the Invincible Sentinel raid boss.
There are, like all the other games in the Borderlands franchise, a ton of farming methods for things all things beyond XP (XP included.) SegmentNext has a great article about a few methods for your various needs.
https://segmentnext.com/2014/10/23/borderlands-the-pre-seque...Courtesy of YouTuber "HandsomeJackBoy" there's also a pretty good and current solo farm method you can employ from this video here.