Shoot the shiny airplanes and collect every one of the little gray crystals that they release. Whenever you collect one of the gray crystals, you will be rewarded with a higher firing rate. This makes it far easier to destroy every enemy on the stage and to do a lot of damage to the larger enemies.
Two of the upgrades that you can buy in the store fire automatically. One of them is the wing cannons. This adds two shots with a fairly low firing rate, but still high enough to make it significantly easier to kill smaller enemies. The other one is the homing missiles. These fire intermittently, and do a large amount of damage to big enemies, as well as help you destroy the towers and other high-health targets.
There is no outward change when you upgrade something that you already have, such as the main cannon, right up until you do the final upgrade to it. Once you do the final upgrade, the bar will reset. The appearance will then change, and then you’ll have another bar to work off of, allowing you to do more and more upgrades.
When you’re going back to unlock medals, start with the earliest stages, because they are the easiest ones to beat. The most difficult medal is usually the one where you have to go through untouched. For this one, don’t focus on any other medals except for this one. Focus only on avoiding shots, even if that means you miss a small percentage of stars.
The easiest medals to get are the ones where you rescue all of the people who are stranded on the islands. Just make sure to keep a lookout for the people because if you see them too late, or you don’t even see them at all, they are very easy to miss.
Want to play without the ads? Turn the phone into airplane mode or shut off the wifi and the cellular and you can pull it off for free. Otherwise, if you purchase the ad-removal in the store, then you’ll get rid of the ads without having to shut off your data.