There is only 10 minikits in the Dark Knight level; the Adam West in Peril doesn't count towards this achievement. When you have collected them all you can save and quit and the achievement will unlock in the recap screen.
Their locations are as followed:
1) Minikit #1: At the start of the level jump to the left, onto the hood of the truck, for a minikit. *Attainable in Story Mode*
2) Minikit #2: At the start of the level shoot the gold lego bricks on the right for a minikit. *Only Attainable in Free Play*
3) Minikit #3: When the second flatbed of police comes in there will be a minikit on it, jump over and grab it. *Attainable in Story Mode*
4) Minikit #4: Hidden behind the big yellow truck in the next section is a minikit. *Attainable in Story Mode*
5) Minikit #5: In the bottom right corner is a magnetic fire hydrant, use your magnetic powers, then freeze the water (1/3). Do the same thing with the hydrant in the top right corner (2/3) and the top left corner (3/3) *Only Attainable in Free Play*.
6) Minikit #6: In the top right corner of the area you will see a silver door with some Harvey Dent posters on it, just to the left of the magnetic fire hydrant. Above the door is a silver potted plant stand, shoot it and then dig up the spot that appears. Put the pieces together and you'll have a minikit. *Only Attainable in Free Play*
7) Minikit #7: On the top wall you will see a blue and white canopy. Fly above it and you will see a minikit on a ledge. *Only Attainable in Free Play*
8) Minikit #8: Head into the alleyway in the top middle and on the right is a grate you can go through for a minikit. *Only Attainable in Free Play*
9) Minikit #9: In the top left corner there is some fire you can put out, do so then build and use the Lantern point and grab the minikit. *Only Attainable in Free Play*
10) Minikit #10: In the bottom left corner is a car with glass windows that you can break, do so and grab the minikit. *Only Attainable in Free Play*
Credit to HappyThumbsGaming for the videos and text.