Battle Ring (Mario & Luigi: Dream Team)
From the Super Mario Wiki
The Battle Ring is an area in Pi'illo Castle that Mario and Luigi can access in Mario & Luigi: Dream Team. The area serves the same purpose as The Gauntlet in Mario & Luigi: Bowser's Inside Story, in that Mario and Luigi face tougher versions of bosses already battled.
Normal X Bosses
Giant X Bosses
Names in other languages