The smaller the size of the signs that you draw, the better. This will save time, and will be noticeable when your screen becomes crowded The faster you are, the better. When facing Bosses with multiple balloons, try the easiest signs first. Usually monsters with more difficult symbols will appear soon afterwards and you will begin building combos. However this requires speed.
Super spells
After unlocking super spells such as Slow Mo or Lightning Strike, they will randomly appear. Do not use them immediately. Instead, wait until you are facing some enough enemies that they will be useful. You can use the super spells until reaching just before the right side.