Designing an American Gladiators-type encounter

My players have returned to a resistance group hideout they'd once done contract work for, only to find the place cleared out by the Empire, who have placed traps specifically for the party. (Traps were thought of, but not designed by, a different group of players in a one-shot, where they played Empire officers looking for the main party.)

One of the encounters is an American Gladiators style obstacle course with auto-cannon, taking place in the training area/gymnasium. The cannon will fire automatically at a creature it sees, on their turn, if they fail their various skill checks to hide, move silently, tumble, etc. At the end, there is a sort of control panel-looking unit that is actually a mimic.

I think I intend to run this as a sort of 4e skill challenge (which I use all the time). The party is a very strong level 6-7 and the purpose of the trap is to kill-or-capture. I want some of the ammunition to be "Resilient Sphere," but that should be the sorta final punishment, if they've failed the skill challenge.

What other sort of ammunition should this auto-cannon have? What other obstacles should I emplace in this gymnasium? (The Empire does the magitech thing, called Alchemerald.)