Long story short, my party of 6 second level PCs (3.5e) broke into a prison tower in the middle of a bustling city, discovered that the prisoner they were trying to rescue wasn't there, and now they're about to escape via the garbage chute into a river below. The river flows out of a grate in the wall, and their plan is to blow the grate open and escape.

The whole city is currently in chaos due to riots caused by a local temple of monks. The PCs asked the monks for a distraction, and they took it a bit too far, and began overthrowing the current government.

Now that we've got a little background, I need some help with designing this encounter. My plan is that it will take a few rounds for the explosive charge on the grate to be set, and the PCs will have to fend off guards while they wait. Once the grate is open, the monks will leap in and fight off the remaining guards, allowing the party to escape.

This will almost certainly be the last encounter in this city, so I want it to be rather memorable.

The two main problems are that the party is rather low on health and healing magic, and that I'm having trouble thinking of unique enemies (who are also thematically appropriate). That's mostly what I need advice on, although I'd love any other ideas that you might have.