There will be a place in the first cave on the alien planet where a big ball rolls towards you. Allow it to hit your character; you will not die. If you have 6 HP, fight monsters and let them hit Gohan for at least 1 HP and do not die. If you have 5 or less HP, use a HP MAX card to get a level-up. This also may be done with Dende. Note: Your BP, HP and KI will increase but not your Exp. level.