
The level 50 achievement is unlocked if you join someone elses game on the level 50 character, I was level 13, jumped into a friends game and swapped to the 50 and unlocked both the level 25 and level 50 achievement. The same thing happened for my room mate.

I'm not sure if this exploit is repeatable. if it was it would be all over the place. hence I doubt he'll be able to give you a 48 or 49 char.
Just level up yourself
If you want the quicker way then there''s the fleshsticks exploit
this is great!, but a question does it ever max out? badass ranks that is
"You are not special. You're not a beautiful and unique snowflake. You're the same decaying organic matter as everything else."
Don't know if anyone post this but this save does not unlock achievements. Just place it in your borderland save file and select the save/ character and do the glitch.
Sent from my Samsung Galaxy S lll
Yeah, no achievement pops. Works great!