It's in the glass showcase at the starting area. Use Sonar (e.g. Black Bolt) to destroy it.
Minikit 2
Go to the right into the fireplace room. Then fly up to the front left corner. It's hovering there...
Minikit 3
Same room, use Single Rope Pull (e.g. Ms. Marvel) on the bookshelf, build the sandpit and use Dig (e.g. Black Panther) to dig up the Minikit
Minikit 4
Same room. Fly up to the balcony with the green goo. Destroy everything and build a therapy bed. The Minikit will hover above it.
Minikit 5
Find and destroy 5 houses of cards
Character in Peril (Stan Lee)
In the hallway, upstairs to the right, there is a box. Destroy everything around it and build a Gravity Mine point. Use it with Star-Lord
Minikit 6
Now going to the library, upstairs, left. Fly up to the balcony, smash the bookshelf and use heat (e.g. Ironman) to open the gold wall.
Token: Clea
In the library, where - in the story - Doctor Strange is tearing down the bookshelf to reveal the Rune platform, there is also a Fuse Box. Access (e.g. Ms. Marvel) it to open the safe.
Minikit 7
Now going up to the display room. There is a Silver Lego box in the top right corner. Blow it with Silver Lego blowup (e.g. Ironman)
Minikit 8
There's a technology panel nearby. It's an elevator. Put another character inside the elevator, then activate it (e.g. Ironman) to get the Minikit.
Minikit 9
To the left at the back there is a gold lego plant pot. Use heat (e.g. Ironman) to blow it up.
Minikit 10
Continue to the boss fight. In the back left corner, jump over the little wall. It's hiding there.