The last level of the game is supposed to have (without risking revealing any spoilers) a boss, correct? I just reached the final level for the first time and I didn't see him. I walked around the room a couple of times, and I could hear him but I didn't see him. So I blew up every green-walled box in the level and still didn't see him. Then I shot the red-walled box a few times with a BFG and I heard what I thought was a pain noise. I ended up winning the game by (again, without spoiling) sacrificing myself. Is it possible that he was spawned within these indestructible walls? This may be a glitch that should probably be fixed in a future release.