Boss Rush

The Boss Rush is one of the "secret" dungeons in the game. Entering it requires an Invitation Ticket. Those tickets have a quite small chance to be dropped when defeating a boss. This includes the minibosses that appear from the second reset on, making it significantly easier to get one. Despite the hint of the developer of the game it doesn't seem to be necessary to use a boss' magical ability against him for a ticket to be able to drop. The Boss Rush's dungeons are built like the other dungeons but you only fight bosses here. Each room contains one random boss. There is no known upper limit in the number of floors.

What makes the Boss Rush special is that you can gain an additional soulbound item here. Only the very first item dropped will be soulbound. Note that you can only get one soulbound item per reset, even though you can get more than one Invitation Ticket. Though there indeed is a way to get more than one soulbound items out of the Boss Rush: If you manage to defeat another boss before you pick up the first soulbound item, both will be soulbound. So you might consider to unequipp the Looter's Ring when entering this dungeon.

Aside from the soulbound items the amount of gold dropped in the Boss Rush is much higher than in any other dungeon. This makes it worth to push forward to higher floors.