There is only 10 minikits in the “Batman 75” 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 destroy the 5 tables for a minikit. *Attainable in Story Mode*
2) Minikit #2: After building the lego pieces from the 5 tables complete the little dance floor puzzle, then use your bat senses to complete the final puzzle and gain a minikit. *Only Attainable in Free Play*
3) Minikit #3: Around the bottom floor and are 4 gold statues, destroy them all for a minikit. *Only Attainable in Free Play*
4) Minikit #4: In the right corner of the room are some balloons attached to a silver base, destroy the base and fly up and grab the minikit. *Only Attainable in Free Play*
5) Minikit #5: In the top middle of the room are four bouncing Batman speakers to destroy for a minikit. *Attainable in Story Mode*
6) Minikit #6: On the left and right walls there are rolled up Lego pieces, yellow/black, that when you shoot them they open up into Joker banners. There are three on the right and three on the left. *Attainable in Story Mode*
7) Minikit #7: In the top middle you will have to build a car wash and transform the Batmobile into the Joker-mobile. Once you do, use your sonar abilities to break open the glass on the front of your car for a minikit. If you complete the car wash last then you will not be able to unlock this minikit so make sure you do this section first. *Only Attainable in Free Play*
8) Minikit #8: In the top left corner use the grapple point and then jump across the poles in the wall to get a minikit. *Attainable in Story Mode*
9) Minikit #9: In the top left corner use the grapple point and then step on the switch to turn the buildings on the right. Next use Harley to climb up and jump on the top right building, then just fall off onto the floating minikit. *Attainable in Story Mode*
10) Minikit #10: Use the grapple, point it in the top left again and use your bat senses to reveal some magnetic Lego pieces. Use Robin to open them and then hop in the building and grab the minikit. *Only Attainable in Free Play*
Credit to Happythumbsgaming for text and video guide.