I have been using this to level up my friends that don't mod or use gamesaves.
It takes about 1.5 to 2 hours to level up someone from Lvl 1 to 50.
I use Playthrough 2.5 with my Commando.
-Fast travel to Tundra Express and go speak to Tiny Tina
-You will have to complete the mission You Are Cordially Invited: Party Prep, First.
-When you get the mission "You are Cordially Invited: RSVP". Get one low level person to stay at Tiny Tina's place and get the Level 50 character to the mission location.
-Clear the area first. Then Flesh Stick will come out. Kill Flesh stick there and it will fail the mission.
(Because you are supposed to lead Flesh Stick to Tiny Tina's)
-Get the low level player at Tiny Tinas to then restart the mission. Then kill Flesh Stick again. Rinse and repeat.
If on playthrough 2.5 Flesh Stick is Level 52 and will give over 5000xp everytime he dies.
Sometimes Flesh Stick will glitch and not apear, so quit the game and start it again.
I use the Commando because of his turrets, they easily kill Flesh Stick. If you have 2 level 50 Commando's in the game take turns throwing out your turrets to save ammo. Its a lot easier and quicker than shooting him everytime. I also equip my cooldown mod and relic so my turrets cooldown is quick.
*Edit: I have Added a Level 50 Commando at Playthrough 2.5 save. It has the 'You are Cordially Invited: RSVP' mission activated and its at Tundra Express.
Click the below link to watch a video on this.
This is Not my video. All credit to the maker. I thought i'd share it here as I have not seen anyone post this info.