Load up a new game on the easiest difficulty and start the first level. Clear the monsters in the level trying not to hit the barrels in the process. Once cleared, head back to the starting room and follow the path blowing up the barrels in the process, DO NOT blow up the barrel at the very start on the ledge until the end as there is an extremely short timer from when you destroy the last barrel to when you get to the teleportal. If you take too long you will miss it and need to do it again.