The Boss Battles mode is a secret, unlockable, mode available in the stadium section of the game. In the Boss Battles Mode, you select a character and difficulty level, and start fighting against all the bosses from The Subspace Emissary in a row:
Note that if you need more information about any of the bosses, just go to their respective pages for in-depth info.
They all appear in random order, except for Tabuu, who is always last. As in All-Star mode, between battles, you'll visit the Rest Station, where you can see who's up next. There are three Heart Containers there that will refill your health completely. And you'll need them, because you only get one life, and there are no continues.
This mode can be played with two players, in which case the Rest Station will have five Heart Containers. The worst thing that you should know about is that if either one of the characters gets KO'd, be it the 1st or 2nd player, the mode will end.
To have the whole mode balanced, the bosses will have about half their regular amount of health and power than they would in Adventure Mode. This may not seem like much on the lower difficulties, but if you're playing on Hard or higher, be thankful of that.
Some of the bosses will do their attacks faster/longer depending on their attacks as the difficulty increases. For example, regular Ridley's ram attack in the sky (a single ram that has a powerful knockback ability) is usually slow on Easy (can take around 3-5 seconds to go from left to right or vice versa), but on Intense, he goes extremely fast (less than 1 second to travel same distance).
No matter what difficulty you play, the bosses get harder and harder as you defeat more bosses. So try to be more careful when playing (unless you are a professional, in which professionals don't need cheats).
The shield and dodges does wonders in this mode, so shield and dodge often. The two best are the downward dodge and the aerial dodge. Do an aerial dodge for aerial attacks, and downward dodge for those near-misses.