To build a high Badass Rank and earn all the perks associated with it, make a pair of new characters in co-op mode and have player 1 beat player 2 in 50 duels. This offers 166 Badass Ranks. Save and quit, then start another new game, creating another new character for player 1 (leave player 2's character alone). Fight another 50 duels with player 1 winning them all for 166 more Badass Ranks. Repeat as desired, and delete the pile of unwanted characters when you're done. If you set the controller configuration to something where you can just keep tapping the melee-attack button without ruining your aim at the other player, you can end a duel-run like this in roughly 10 minutes. So in one hour you can build up 996 Badass Ranks.
Once you complete the quest "A Dam Fine Rescue", you will find Sir Hammerlock inside Sanctuary offering a quest called, "Mighty Morphin'" that involves taking an Injector and injecting Varkid Cocoons with a special serum. Injecting the serum into a cocoon forces a huge flying Badass version of a Varkid. Killing one yields 1,900 Experience. If you don't complete or turn-in the quest you can wipe the entire zone of cocoons, save the game, reload, and repeat.
Once you reach Tiny Tina do all of her side missions until you make it to the final stage of her tea party. Here she wants you to lure Flesh-Stick to her little cove to join her tea party. Since you can fail this mission by killing him, you can actually repeat it. If you have a second controller and another character, have that created person join your session and have them run all the way to Tiny Tina. If you keep them in front of her and your main character in front of where Flesh-Stick walks out you can kill him and re-accept the mission as fast as your trigger finger allows, making anyone reach about level 20-21 in a matter of minutes.
Begin the "Winter Is A Bloody Business" quest, which sets you out to defeat an evil prince named "Jeffrey", a reference to Joffrey. Defeat the guards in a throne room that resembles the one in King's Landing, two guards named "Canine" and "Molehill", a reference to the Hound and the Mountain.
First, place the desired item(s) on a character, then ready or create a second character to take on as many of the item as you want. Start a two-player co-op game with the first character as player 1 (and the other character as player 2, obviously), then give the desired item(s) to player 2. Have player 2 save and leave game, then quit the game through the PS3 system menu. Since the game doesn't save player 1 when player 2 leaves nor when you quit from the system menu, both players will then have said item(s). Repeat as desired (for faster duplication, transfer all copies of the item to player one, save-and-quit with both, then start over with the duplicates on player 1).
In the Tundra Express, complete all the optional quests for Tiny Tina that lead up to "You Are Cordially Invited: RSVP". This quest requires you to lure Flesh-Stick into Tiny Tina's cave so that she can put him in a trap. The trick is to just kill him. Depending on what level you are (Level appropriate on first playthrough is anywhere from 1800-3000 exp per kill, playthrough 2 is anywhere from 2000-4500 per kill). After you kill Flesh-Stick, simply go back to Tiny Tina and restart the quest. If you are online and playing with friends, just have 1 friend stand by Tina while someone else stays by Flesh-Stick's spawn, and he will pop up instantly each time.
Two class mods are needed for this to work. When your backpack is full, switch your currently equipped class mod for another one in your backpack (you can then re-equip your original class mod after doing this). Then, exit the backpack menu. You can now pick up one more item and have a backpack with 28 items in it instead of maximum 27.
Patricia Tannis will give you a quest called "Splinter Group". The first step is to pick up some "bait" from Mad Moxxi, which is actually pizza. Patricia will say after you pick it up that they love the stuff. You end up going into the sewers, which then is a hideout of the runaway mutants. Leave the pizza on the table and get ready to fight four "brothers" who talk like the TMNT turtles. After defeating them, you will go further and can choose to just leave and forget about the puzzle in the final room, or try to complete the puzzle to turn five red lights into five green lights. Once all lights are green another mutant will will come out of hiding and attack you. All five of these mutants does flips, rolls, and jumps just like a ninja would do.
Enter the Konami code at the title screen: UP(2), DOWN(2), LEFT, RIGHT, LEFT, RIGHT, B, A, START. This will unlock Extra Wubs in the gameplay part of the options menu. Extra Wubs literally does nothing.
At the Sanctuary Hole after Chapter 9, jump off a cliff and go to Caustic Caverns. Run northwest to Guardian Ruins then go to the minecarts and turn right. Jump over the rectangular rocks and turn left to find Minecraft dirt. Break though it to reach Minecraft stone, cobblestone, and coal. Break though those to find Creepers along with blocks of gold, diamond, iridium and more. Kill the Badass Creeper to unlock Minecraft-themed skins (Steve Head, Steve's Clothes etc.).
Accept the quest "Rocko's Modern Strife" and go to Thousand Cuts to start "Defend Slab Tower". You should notice that no foes will spawn in the area as long as you don't progress through the quest. Since normal foes will no longer spawn, you are free to circle the area and loot every chest without being hindered by enemies. However, foes such as one-armed bandits and loot midgets will still spawn. If you save and exit after clearing all the chests, you can just accept "Defend Slab Tower" once again and clear them out as many times as you like with the same conditions as long as you don't complete the quest.
"Not Full Of Monkeys" is a challenge that invloves killing enemies with stationary barrels of any type. With Gaige the Mechromancer DLC, you don't need the barrel! Anger a swarm of Rakks and summon the Deathtrap. When they rush you, the Deathtrap will use its laser to kill them. It counts as a stationary barrel this way. Try this on flying enemies for some quick Badass rank.
The "Lost Souls" side quest contains many references to Dark Souls. An undead hero named "Crestfallen Player" is waiting near a bonfire and states that he's died "eighty times" and another player keeps invading his game to steal his souls. Once the souls are retrieved a glowing red knight named "-=n00b_killer=-" will appear. Kill him and the Crestfallen Player will say that "-=n00b_killer=-" repeatedly backstabbed him while he drank from his flask. Once the side quest is over, he says "It can be hard out here on Pandora. You'd better hold onto your humanity as best you can."