This is a solution for the boulder and barrel puzzle just before the end. Don't read it if you want to solve this for yourself.

Both balls => NE to wall
Both balls => NW to wall (one will be on moving plate)
Bottom ball => SW to trigger
Use the barrel to jump from to push ball from the moving plate to the platform.
Push the barrel on to the other trigger
Push the ball to the other moving plate explode the barrel.
Push the ball over and down to the final trigger.