World 4 - Level 5
This last remaining UFO is quite dangerous. You have to time your steps carefully to protect both Peegue but also all four wooden boxes in the lower lower corner. As soon as the UFO passes, run along the yellow path. Use the three boxes to build a bridge at the "X"s (again, time the UFO). The final box has to be brought out and around, again using the yellow path. It is imperative that you (a) wait until just after the UFO has passed again and (b) follow the yellow path exactly - do not push the box onto the yellow button yet as this will cause the entrance to remain open long enough for the UFO to fire a shot at it.
Once the UFO is clear once again, push the box on the yellow button, proceed to the purple button, and release the cannons to destroy the pesky UFO. And that should be all there is to it, shouldn't it...???