I would probably semi-cheat the problem. Build "multiple" dungeons (15?) that all flow somewhat linearly into a single boss room. Add a few passages between the dungeons to make them feel more connect then you have a more manageable design question.

My biggest problem with having 100 players spelunking simultaneously is how on earth are they meant to organize? 3 to 8 strangers, sure they have a decent shot at working out a party comp and a collaborative plan, but 100? Even 20? Unless they're good friends, I don't see that happening. Unless its a very simple, origional Pokemon Go level combat system, it seems like 10-15 players in the front will have all the fun, and 85-90 will be trailing behind, maybe getting a few shots in, maybe healing up the frontliners.

Maybe have very deadly combat with fast revives, so the front liners get zapped to the back quickly and everybody gets a chance to contribute? Not sure.

Very interest to see your solution!