Enemies
Notice that some aliens are lighter than others. The lighter the alien, the weaker it is. When waves become nonstop, focus on those weaker aliens to force your way through.
Minions
You will start the game with 10,000 gold. Spend 5,000 gold on your first minion. Minions are small companion planes that fly with you and add extra firepower. They will earn experience and become stronger as they are used.
Special attacks
The slow down power-up can be activated at any time by tapping the downward arrow at the bottom right corner. This can only be done once per run, but can be very useful. It slows time for everyone except for yourself. Use it if you become cornered or when the screen is filled with bullets.