Your priority should be to get rid of bubbles which contain horizontal arrows facing opposite sides.
You can start by removing arrow bubbles on the left side of the screen. You will notice that arrow bubbles are clinging to green/blue bubbles. Target those bubbles to make arrow bubbles fall on the ground. As you do this, don't forget to remove black bubbles. They too will be hanging from green bubbles and so you must target such bubbles to get rid of a bunch of black bubbles. Make the most of the aiming line and look for those bubbles that hang from green or blue bubbles.