Boss Rush is unlocked after getting the ending where you sacrifice all the other adventurers, unlocking
In case you were wondering, this is done by simply killing them as soon as you meet them, don't talk to them at all. It can then be accessed from the main menu, not from the modes list since it does not have a save file of its own.
Before you select the option, you can hold
to activate cheats.
actually makes the bosses harder, so instead just hold
. This will make it so you get to play as Ultimate Zangetsu, with his charge attack and double jump, and also gives you infinite weapon energy. Very handy, and it does not affect the achievement. Thanks to Jakez123
for tipping me off on this.
You will predictably enough be fighting each boss in order, and you can choose subweapons for each character in between fights, as well as heal with two big hearts. Your subweapons are reset after every fight, so make sure to pick them up again even if you don't want to change them.
The bosses all behave as normal, which is to say they are not the upgraded Nightmare/Ultimate versions. They do however do a bit more damage than normal, and you don't have any health upgrades, so keep that in mind.
Assuming this is one of your last achievements, you should be very familiar with all of these bosses. The hardest ones are probably the second and last boss, but with infinite weapon energy, you can spam most of them to death before they can do much. Miriam's triple dagger makes very short work of just about everyone if you mash the button, and if you activate Albert's lightning magic beforehand and let it fly around and do its thing, things will go even faster. His fire shield is great too if you just want to be safe. Zangetsu's fire buff power combined with his charge attack also downs some bosses very quickly, especially boss 5 and 6. As usual, if one character gets close to death, switch to a different one, and make sure you heal the right people between fights.
If you have any questions or need advice about any specific boss, feel free to leave a comment. Good luck!