"I may not be the King, but I sure am BIG!" —Big BooBig Boo is a mini-boss in Luigi's Mansion 2. He is very similar to Boolossus from the first game in the regard that he is a colossal Boo that is made up of many smaller Boos. He is fought in mission "A-4 Ambush Maneuver", where he traps Luigi when Luigi thinks he is going off to fight King Boo.
Big Boo is (as the name implies) a big boo. His appearance does not change from that of a regular boo, besides the fact that he is much bigger.
After shining the Dark-Light Device on the main train part. Big Boo will appear and "introduce" himself. When the battle begins, Big Boo will attempt to smash Luigi into the ground three times before getting dizzy, or he will sink part way into the ground and charge into Luigi. When he's dizzy, this allows an opening for Luigi to suck up his tongue like any other Boo and shoot him in a direction. In this fight, Luigi will want to aim for the drill on the front of the train that is circling the arena. If he charges, Luigi can stand in a way so that Big Boo will run into the front of the train.If he hits the side of the train, he will bounce off and be stunned for another chance at shooting him into the drill. Either way will pop Big Boo into many smaller Boos, much like the fight with Boolossus in Luigi's Mansion. Next, Luigi must find and fire the small Boos into the cages that make up the rest of the train. The longer Luigi charges up his Poltergust 5000, the farther they'll fly, however if he waits too long, the Boo will charge into Luigi and escape his grasp. There are ten Boos in all, and each time one is captured, Big Boo will become smaller. Eventually they will all reform into Big Boo. If Luigi shines his flashlight on Big Boo it will stop him in his tracks, however eventually he will disappear to escape the light.