While destroying enemy units is important, keep in mind that your priority should be your plane's well being. Taking down an enemy means nothing if you get a colossal amount of damage. After you get to x20 combo multiplier, you can back off a little since it's the maximum and you can't chain any more. When you encounter a huge cluster of enemies, you can usually get rid of them by shooting a nuke that is nearby. You will need a couple of shots to make it explode, though. If your inventory is full of devices, you can simply use the black box to combine your items and, not only free up the space, but also get an upgraded device. Sometimes you don't need to pay that much attention to enemy ships, especially if it's a boss fight. You can simply focus on your plane and the bullets that come near it.