Fly on the cute little dragon and kill all enemies that will drop gold on death. Pick up gold and stars to maximize the income. Buy important upgrades that will improve your fire rate, the damage or the amount of bullets shot at once.
This is the real hell! You've entered the realm full of mysterious creatures and while you proceed further their quantity will increase. Kill enemies and collect the dropped gold, which can be used to install some important upgrades. Purchase more powerful gun and increase the fire rate. Show your enemies that you are the one they shouldn't deal with!