"Boss Fight" Ideas

[ApeCake]

So my map is near completion, but I want the end to be special. I want to implement some sort of boss fight. Let's just say it right off at the start, Abductions boss fight was amazing. Fun challenge and greatly executed. The particle effects made it complete. However, the story in my map doesn't have a "bad guy". So a direct boss fight like in Abduction wouldn't work. The monsters are mentioned though, so I want it to be about the monsters. My first thought would be some kind of tower defense (yes, I am trying a lot of custom weird puzzles in my map). Although it wouldn't be impossible, it would be really hard to pull it off. My thoughts were things like this: There is a hallway with the sewerspinthing in the middle of it. If you pull a lever, the sewerthing will start spinning. If a monster spawns (it would spawn based on time) (it will spawn on the opposite side), it will walk towards the player and will walk into it. The monster will be set inactive, and blood particles appear. That is just one idea of the "towerdefense" kill machines. However, I do not think this will be fun or hard at all, mostly confusing. I have no idea what else I could create at the end of my map. I do not want it to be "click on the level door that says "outside" and the credits start", but I want the end to be new and challenging. Any ideas?
No ideas are crazy enough, any ideas are good. Notice that I will be using Amnesias models and entities only and won't be using custom-made ones.

Sorry if there already was a thread like this posted before.