Here's some info for those who
do not want to use an exploit to get to level 50.
First, when you beat the game for the first time you'll be given the option to start a game in True Vault Hunter Mode. This mode allows you to play through the game again with your same character, but with tougher enemies that give you better loot and exp. Feel free to do so. You can switch between True Vault Hunter Mode and Normal Mode at any time from the main menu by pressing
to go to character select. You'll be able to choose your character, and then which mode you want to play on if you've beaten the game at least once.
Some things that DO stay with your character between modes:
Your level, loot, equipment, skills, and challenges travel with your character between modes.
Some things that DO NOT stay with your character between modes:
Missions will have to be done exclusively on each mode. You cannot beat a mission in one mode to have it counted in the other modes. This is important when going for "Complete all missions" achievements, as they must be
all be done on the
same mode.
What I recommend is first beating the game on normal. If you're simply going only for this level 50 achievement, then switch to True Vault Hunter Mode as soon as you beat the game.
If you're going to try completing all the sidequest, map exploration, and miscellaneous achievements you should do those on normal difficulty. They will generally be easier and faster to complete than on any other mode. There will be a few quests that are tough to beat by yourself on normal the first time around, namely the Hyperion Slaughter, Round 3 Torgue, the Badass Round of Murderlin's Temple, and raid boss (invincible) missions. What you can do is switch to True Vault Hunter Mode, level up (to 50), gear up, and switch back to normal mode to mop up those formerly difficult quests.
If you think I need to add anything please let me know in the comments. Thanks.