There are two or more enemies are in alignment with each other!? Tap on the enemy that's furthest away that way she'll blast through the whole lot of them, giving you a combo-boosted score.
Keep in mind that if you tap on the screen that isn't directly over an enemy, you'll lose some of your energy meter.
You'll also lose energy for any enemies that travel off the left-hand side of the screen.
Make sure you pick up the energy upgrade potion to compesate lost energy. With it you'll be able to tap on potions that pop-up on the playing field.
The Shield potion will stop for a short time any enemies from disappearing off the left-hand side of the screen.
Take out as many enemies as possible for more gems. Each one you destroy will add a gem to your score.
Use The Freeze potion upgrade to stop everything on the screen dead in its place. After that try and take out more than one at once so you get some decent combo points.