Best way to do the combo: after you hit the shell at the very beginning, wait until it stops. Then jump on it, so it goes up in a straight line, and every time it goes down jump a little and hit it with your head to mantain its vertical course. Not so easy anyway, but seems to me the best method. Hope it helps
This is how a sequel should be done: new challenges, lots of upgrades that now you are able to transfer on your new ships, and even the choice to use the mouse, so you can aim your shots. Great job!