Once you're back on the ground floor, you'll need to circle this room from above, then escape through the grate-less passage on the left. Afterwards, turn left; another inactive chamber will be blocking the other path. Squeeze through the shelves. If the monster was still after you, he'll stop now. Ahead, by the corpse of the dead guard, you can find a battery, and on the right there's a collapsed passage you're going to have crawl through. Afterwards, follow the corridor until you reach the arena entrance where one the bad guys beheaded a man.