First, move all the way to the left, and destroy the box that's directly underneath you. Following after that, destroy all the box on the left and go down, and release that mummy. Once he comes out and move in your direction, jump over him and go in to take the key. Remember to make a box to block him as you are trapped in, you can't escape if he comes near you. After that, go out and make boxes and go to the ledge above where you started at.
Remove the most leftern box where the naga is standing, so that he'll fall to the most bottomost of the stage where the mummy is contained. Hence after that, destroy all the boxes where the naga was supposingly standing on so that the second mummy have a larger flight range. After the second mummy flies to the left, quickly hammer the blue flame to minimize it. Jump over the flame and create a box to prevent the mummy from coming, thus containing it.
After that, head up and as the 3rd mummy is moving left, block him with a box, thus containing him in the left part of the area. Meanwhile go and destroy all the boxes that limit the flight pattern of the topmost mummy. Destroy the box which contains the 3rd mummy when the 4th topmost mummy is on the right, so that you can jump over the 3rd mummy without worrying that you hit the 4th mummy and die, and than you win the stage.