the trick is not to step on the "fire" panels , because in that case the bridge you're standing on will retract.while on *easy* difficulty level you have enough time to go around those fire panels, on *hard* you will run out of time very soon (given the fact you also need some time to shoot at those objects on the wall).thus, what OP has also been doing is that you simply jump over those fire panels (altho, every time i've tried to do it, i'd trigger the fire panel and fall in the pit )