Captain Boo Trial
From the Super Mario Wiki
This is the fourth of seven bonus levels that involves the captain battling enemies by themselves. Captain Boo is the selected captain to enter this level.
There are two battles in this level; the first has two Parabeanies, a Piranha Bean, and two Beanerang Bros in the front lines, as well as an Elite Troopea captain. The next one has two Parabeanies, two Piranha Beans, and three Beanerang Bros, as well as another Elite Troopea captain.