Fast leveling and easy experience farming

What is the sweet spot for experience gain? Is it more effective to fight the highest level enemies I can find, or am I penalized for being out of my depth? Are certain areas of the game better/best for experience gain/leveling? Are bosses better than areas with infinite minions? Would I get exp faster in a higher-level co-op game? What are the most time-effective techniques for rushing/boosting a character (either solo or co-op)?

Kill flesh stick again and again in the Tina special guest quest. best if your coop partner stays by her and keeps beginning the mission while you are where he spawns waiting for him with a shotgun or else. at lvl 14 its 1800exp every time. i leveled to 20 in 5 minutes.

Note: If you restart the quest too quickly fleshstick will stop spawning and you will have to reload the area. To avoid this, wait until Tiny Tina finishes her line ending with "my abode". If you mess up, first she won't speak her next line. If this happens, wait a minute or two (to let Tina finish the sentence, even though you can't hear it) and start the mission once more, Tina should talk again.

What I found interesting is if you kill one of the 3 Rakk flying around in vault of the warrior one will continuously spawn from a spot around the arena flying at you and attempting to attack you.

This becomes quite awesome because they do not stop coming at you, if you're prepared to stand on the spot (at the bottom of the stairs just before you fall into the arena is the best) for a while, it's good xp.

At level 50 playing TVHM they give you an average of 300 XP per kill. If you have adequate power in your shots or a good gun with a decent size mag they will drop like a rock. If you have the bee one hit or if not maybe from 2-3 or 5-7 depending on power of gun. This becomes around 3000 to 6000 xp if you can kill 10-20 rakks, in one magazine. So after only 4 mags you get the same xp as if you killed the warrior.

This is great for boosting another character, just throw him in your game and kill rakk with your lv 50 guy and xp is yours. It is a bit time consuming but not as much as other methods, and it's damn easy. About 20 minutes to get lv 47 commando to lv 48.

Hope this helps :)