Seriously Good At This - 40
Complete the game on the hardest difficulty setting.
This achievement is really easy. You can play on the difficulty Easy until the final area where you fight Atlas. This method can be found on easily on the internet. I see there are 2 Guides for it here, but they aren't very detailed so I will write one in my own words. You can also get "Brass Balls" but you have to have the DLC. Make sure you read the IMPORTANT section (If going for "Brass Balls") and do the following:
1. Start a game on Easy, and play through (you can go for any achievements you want while you do) until you get to the very end of Proving Grounds (The second last area). You will be a Big Daddy and you will be with a Little Sister. When she climbs in the hole and give you her weapon to steal ADAM, you are at the end of the game.
Note: Do not switch the difficulty at any other time in the story, because it may affect you not getting the achievement. Read the comments below for more information.
2. This is only if you are going for "Brass Balls". Press Start and go to Options. At the very bottom disable Vita-Chambers (it would be wise to disable them in the very beginning of the game; read the IMPORTANT section for more information).
3. Go into the Elevator and you will Ascend. You will then be in a new area "Fontaine". This is where the Boss is. Fight him normally, draining his ADAM. You have to drain his ADAM four times.
3. When you go to click "A" to drain his last bar of ADAM (the 4th time) after you click "A", wait half a second and then click Start. Go to Options and turn the difficulty to Hard.
4. Now this is VERY important. When you turn it to hard, click START! Do not click B to return. Make sure you click Start.
5. After a few seconds you will go into a cut scene. At one point in the Cut-scene you should get the achievement. If you are going for Brass Balls that achievement should unlock too.
6. Wait for the whole cut scene to finish. If you don't have the achievement, load your game up from the last auto save and repeat steps 1-5. You will be coming out of the Elevator.
IMPORTANT: Make sure you do the following because if you don't you MAY not get the achievement "Brass Balls".
1. Don't Die in the game. This should be easy on Easy. Unless you disabled Vita-Chambers from the very beginning, you should be able to die and still get the achievement. But to be safe, try not to die.
2. Make sure you have the DLC and you are connected to xbox live (you need the update).
3. Make sure you Disable Vita-Chambers before going up the elevator. (Even the smartest people make mistakes)
4. Make sure you change the difficulty to "Hard" when you are Draining his last ADAM bar.
5. Make sure you press the START button after changing the difficulty. Do NOT press "B".
Credit goes to many people throughout the internet who have confirmed this, friends, and myself as well!
Enjoy the easy achievements!