From Galaxy Attack: Alien Shooter Wiki

Bosses are enemies that boast a lot of health, use special attacks, and require skills along with trial and error in order to beat. Bosses appear on the last wave (usually wave 10) on every 4th level in Campaign mode. In PvP they appear on every 6th wave.

They all have very different designs, but they share some attack patterns. One thing in common about most of them, is they slowly move left and right. Bosses are more rewarding when you defeat them than other levels before it. You can also fight bosses that you already discovered by going to the Boss tab and choosing the difficulty. This feature was added so players could practice the boss's attack patterns to dodge them. You will start off at max bullet level when using this.

The only boss that stands out from all of the other bosses is the Soklo Guardian, as it is the actual first boss that the player will fight, and there is no way to get around it, in other words, no way to skip it.

When you kill a boss, you become invulnerable from any of that boss's weapons for approximately a second. This allows you get to safety once you kill it. Be aware that after this time, anything remaining on screen will become lethal again.

Intro Boss

Wave Bosses

Mini Bosses

Special Event Bosses

World Boss

Clan Boss