I had the same problem you had a few times and after discussing a few years back with a very kind GM he explained what happens when you get stuck with "Fight Your Way to the Arena of Power"
What happens is if you kill the any of the Bellmen along the hallway Before they ring the bell you will not get the next spawn wave of mobs.
The GM explained basically that they are not authorized to run / perform scripts within the game..... Apparently only deceloppers have that authority. This particular mechanic in the final hallway is purely script based and the GM's have no possibility of helping adventurers in completing the quest.
At the time I asked the GM if he could simply teleport us into the arena as a workaround. Even if he did that the bosses would not spawn as the script needs to be fired by killing all previous hallway mobs.
Also what is highly probable you may have had a mob run into the wall and this also would make the quest bug out similarly to the above mentioned scenario.
Hope this helps understand the frustration you may have at this time. It sucks when it happens but it is purely out of bad luck this occurs or when you nuke the bellmen too quickly unfortunately.
Laters and dont give up.