I'm not sure exactly which puzzle you mean but there is one where there are a whole bunch of echo platforms and down the bottom are three together that you need to get to. I think that's the level 7 puzzle. I think you just have to remember that you only need to get one echo down to the bottom and then change to Ben and then back to echo. Multiply echo and cover the three switches.
If that's not the right puzzle then sorry but I'm not near my XBOX at the moment.