it's been a while since I played the game, so if nothing really changed at all (spoiler):
when you get floor 25 for the first time, the boss will kill you instantly, so you need to play the game again.
when you get floor 25 for the second time, now you will be able to fight the boss.
and if you manage to beat the boss, game will ask you whether you want to end the current run or enter the endless mode.