10) Go back to old levels and start them over again to farm for gems.
Old sectors will always give you similar amounts of gems no matter how many times you play and beat them, and no matter how much easier they end up getting due to the numerous ship upgrades you’ll likely engage in. Go back and farm the crap out of them for gems. You can do this without having to use any lives either, since they’ll be easy to win. Do this if you want to earn lots of equipment, too.
9) Start back over from the very beginning level to make your gem farming as lucrative as possible.
The tip in the first paragraph, but on steroids because level 1 ends up being so ridiculously easy when you have good upgrades. You’ll be able to just play right through without thinking, and end up with hundreds of gems in the process. If you do this in the middle of a sector you will lose all of your progress, but if you are stuck on a part of a sector, it might be worth it.
8) Play the secret missions as often as possible
Secret missions will generally earn you more gems (a LOT more gems) than the standard missions will. In addition, you get other bonuses for winning, such as free credits at the end of the level, or a rare cargo box with the price dropped from 3 credits to just one credit. If you lose, though, you won’t be able to play again for about 24 hours.
7) Play the secret mission again immediately
If you want to do this, use the time lapse cheat in order to make it happen, but this time set your phone ahead by 24 hours instead of one hour. Then when you go back to the game, the secret mission will be available again. Keep doing this to load up on the prizes and credits and gems, assuming you’re strong enough to keep beating the stages.
6) Slow down the fight against bosses in order to win more easily.
Each boss has its own weak point (or weak points that switch back and forth) and you have to shoot it until it loses all of its hit points in order to defeat it. Slow down the fight, avoid enemy shots, sweep across with some fire when you can, and eventually you’ll outlast the boss and win. Don’t sit there and aim at the weak point or you will become an easy target for a hail of bullets.