The Final Showdown: All Minikits and the Rescue | Lego Batman Movie Story Pack | Lego Dimensions - Camzillasmom - Reviews - Guides - Playthroughs in 2017
Use a vehicle with "Underwater Interactions" Ability to pull the 3 plugs on the wall under water. In my case, I used Lumpy Car at Level 3 (Lumpy Space Princess' car)
For the story, turn the switch until you get the passage after that bomb filled wall. For minikit 3, continue turning. On the next wall you have to use Suspend Ghost and then Charge up on Charge Transfer. Then turn the switch again and use Charge Transfer to get the Minikit