Cool game. At the beginning it feels very easy but then it get tough. But throughout the game I got used to analyze possible crossings and the last 10 levels were fun.
Ok the simlest solution to lvl 13: After opening the door, place a standing boxman to the rightmost button. Then go to the right of the right lever, send a running boxman to the right so he will fall down and go for the cube. Then go between the levers and watch carefully. At the right moment send one boxman to the right lever and switch the left lever with yourself, so the bottom boxman could go to the finish.
... Need a bit luck but wont take 300 attempts :D