Making a Monster
Built the New Car: Monster
Story-related. You will unlock this after you complete the "Grease monkey" mission.
Story-related. You will unlock this after you complete the "Prison Break: Try Not to Get Shanked" mission.
Destroyed Lance Depot
Story-related. You will get this during the "Loot Larceny" mission
Killed Crawmerax the Invincible
Story-related. You will get this after you complete the "You. Will. Die" mission. See the Mission List post for tips.
Sneaky Little Buggers
Killed each of the loot midgets
There are 5 types of loot midgets for this DLC. You do not have to kill them all, you just have to kill one of each kind. They are as follows:
Meat Popsicle: In the T-Bone Junction, when you are on the mission "Boost the Monster" you will go down a couple of houses on grates. The last house has a locked door and on the other side is a switch. Flip the switch and the door will be unlocked. Inside the freezer resides Meat Popsicle. He is also located in the Prison after you kill Jerome and Mr. Shank threatens you. Go straight, through the door in front of you and there is another Meat Popsicle in the freezer.
Truxican Wrestler: In the prison, Truxican Wrestler is located in a couple of the lockers. I heard that it can be random but I found him to the left as you first enter the prison and in various other lockers throughout the prison.
Mini Steve: Mini Steve is also located in the prison and he is inside of the red loot chests. There are a couple of places that he could be, like the Truxican Wrestler, but I found him by the broken Claptrap. I also found him in places other than the prison. I found him in a chest by Moxxi's Graffiti #1 and found him by the Took Pipe in the "Purple Juice!" mission.
Crimson Shorty: Crimson Shorty hides in the Lancer Chests and can be commonly found during story mission 14. If you are facing the entrance to the armory, turn around 180 degrees and he is in the chest on the left.
Dumpster Diver: During the mission "Little People, Big Experiments," you will come to a room where you face off with Motorhead. After you beat him, on the left wall there is a dumpster. Open it and the final loot midget will be before you.
Destroyed a Lancer while in a Racer
Fairly simple trophy. After you complete the "You've Got Moxxi: Moxxi's Red Light" mission, you will gain access to the Racer. You can shoot and destroy the Crimson Lancer entirely in the Racer (which you first meet on the highway after the above mission) or you can shoot it, get into a Racer, and then destroy it to get the trophy. You can also obtain this trophy if you are in the car while your teammate destroys the Crimson Lancer.
Reached Level 51
Refer to Ding! Overleveled to 11 trophy.
Ding! Overleveled to 11
Reached Level 61
I would recommend playing through the game on playthrough 2 and you will level up much faster than if you play on playthrough 1. Just keep completing missions and killing enemies and you will continue to level up.
Completed all mission in Secret Armory
For this trophy you have to complete all the missions in the Secret Armory. A list of the missions and tips for those I found confusing or challenging is posted in the "Mission List" post.
Sucker Born Every Minute
Paid for a worthless tour of the world's largest bullet
The World's Largest Bullet is located in the Sunken Sea. In mission 11, "Code Breaker: Time is Bullets" you will go there to pick up a communication device. To get the trophy, however, you need to go to the house next to the bullet. On the house there are numerous arrows pointing to a box with a money symbol on it. You need to deposit 8 million dollars and you will get this trophy.