The phrase "boss stage" refers to the final and toughest level or challenge in a video game. It usually involves a powerful enemy or boss that the player must defeat to progress further in the game. Full definition
Well, by default, it's all a bit simple — at least until the very final boss stage — and could really be completed in an evening's worth of play. (thexboxhub.com)
There will also be items in boss stages where players can find strategically placed items to help out in battle. (vg247.com)
There are also boss stages where you'll be racing towards your opponent and jumping onto them. (jgghgames.com)