Bosses are similar to normal enemies except they have more health. Boss battles also have a time limit. If you run out of time, the Boss' health will be restored. To win, swipe it a lot, even if you have a lot of damage per second upgrades.
Gems
Gems are the game's premium currency. Tap the star icon on the bottom bar to access the missions. Complete them, then collect your reward to earn gems.
Gold
You will continuously earn gold, whether the game is active or not. Upgrade the bank to allow it to generate more gold over time.
Movement
When swiping at the aliens, swipe in a circle as fast as possible. This should result in registering a larger amount of swipes and will allow you to attack multiple times without actually swiping in lines.
Upgrades
Main damage upgrades fall into two categories, per swipe and per second. For an active play style, consider purchasing the swipe upgrades because you will be manually doing most of the damage. When allowing the game to idle, purchase the time upgrades.