There should be a boss run gamemode where one can play through a bunch of bosses consecutively. You get 3 pills, 3 remedies, and two sets of three vials of your choice. Since most of the boss music is the same, the background music for each boss battle will be changed so you don't get bored of the same BGM playing for the entire boss run. You only have a certain amount of time for each phase of a boss. If you beat the boss before the phase time limit is over, you'll automatically get crowns. If you can't beat the phase in time, the boss automatically moves on to the next phase, and you don't get any crowns for that phase. If you don't get damaged at all during a successful phase, you'll get a large crown bonus.
You may only revive three times through the whole run. Revives are free, but reviving causes you to lose all the crowns you've earned thus far and gives you all your pills, remedies and vials back. If you choose not to revive, the run will end and you'll keep whatever crowns you've earned.
Between battles you don't heal: You pretty much have to go through all the bosses on the one life, although occaisionally after a phase is beaten, health pills, remedies and vials may drop.

First up you face Snarbolax, of course. This music plays in the background. Snarbolax actually has three phases (when you damage Snarby enough during one phase, he'll automatically Defense Up)
As soon as the Snarbolax's body disappears, your knight will say something cheesy like "The only thing you're eating tonight is my sword" and then the screen will fade to black showing your score and time. A "continue" button appears, and pressing it starts the next battle.

To mix things up a bit, you'll then face the Ice Queen. The music changes to this. To make things a little fairer, the Ice Queen won't have as much health. When she's beaten, your knight will say something like "It's getting a little chilly in here"

Next, you'll face the RRT (Rainbow Roarmulous Twins) and the music will be this. They're pretty much the regular Red Roarmulous Twins, but all their bullets shoot in all different colours of the rainbow.

Now, for some cruel reason, the Bosses start getting exponentially harder. You'll face Margrel to this music, but Margrel fires 4 times as many bullets, and they'll set you on fire if they touch you.

Then you'll face Seerus to this music. The difference is that there are Ghostmane Stalkers on the level spamming sawblades. Seerus has also invented a machine that can freeze time temporarily (allowing the Ghostmanes to throw a couple slow moving sawblades that you'll just be given a very short time to dodge)
Players without the expansion can still play this boss, it'll be kind of a "reward" for getting this far.

Finally, You'll face Vanaduke. The only difference is Vanaduke's mask will always fire those shadowy-red bullets, even when it's attached to his body, and this music will be playing.