Medusa is the first boss of the Invasion campaign. Using a combination of homing serpents, bullet rains from one or all sides, and bulky body shapes; it tests if the player has any of the important skillsets for conquering the enemies of the remaining campaign.
2 Top-Left/Bottom-Right Guns: Turret/Shield if a player has no Shields, 6 Broadside/5 Sweep otherwise
2 Top-Right/Bottom-Left Guns: 2 Cannon/Shield if a player has no Shields, 2 Damage/5 Slowmo otherwise
Core: Spawner/Shield
Form 2
4 Small Guns: Turret/4 Repel
2 Big Guns: 4 Damage/Shield if a player has under 2 Shields, 2 Damage/5 Slowmo otherwise
Core: 20 Bumper/Shield
Form 3
2 Leftward-Angled Small Guns: 2 Cannon/2 Freeze
2 Rightward-Angled Small Guns: Turret/4 Repel
2 Left/Right Big Guns: Carve/Shield
2 Top/Bottom Big Guns: 4 Barrage/2 Erase
Each Xulgon Minion: Cannon/8 Dash if a player has no Shields, Airburst/2 Slowmo otherwise
Easy Mode
Form 1
2 Top-Left/Bottom-Right Guns: Turret/Shield if a player has no Shields, 8 Broadside/5 Sweep otherwise
2 Top-Right/Bottom-Left Guns: 2 Cannon/Shield if a player has no Shields, 2 Damage/5 Slowmo otherwise
Core: Spawner/2 Shield
Form 2
4 Small Guns: Turret/5 Repel
2 Big Guns: 4 Damage/Shield if a player has under 2 Shields, 2 Damage/5 Slowmo otherwise
Core: 20 Bumper/2 Shield
Form 3
2 Leftward-Angled Small Guns: 3 Cannon/3 Freeze
2 Rightward-Angled Small Guns: Turret/5 Repel
2 Left/Right Big Guns: Slice/Shield
2 Top/Bottom Big Guns: 5 Barrage/3 Erase
Each Xulgon Minion: 2 Cannon/Shield if a player has no Shields, 2 Damage/4 Slowmo otherwise
Nightmare Mode
Form 1
2 Top-Left/Bottom-Right Guns: Turret/Shield if a player has no Shields, 4 Broadside/5 Sweep otherwise
2 Top-Right/Bottom-Left Guns: 2 Cannon/Shield if a player has no Shields, 2 Damage/5 Slowmo otherwise
Core: Spawner/Shield
Form 2
4 Small Guns: Turret/4 Repel
2 Big Guns: 4 Damage/Shield if a player has under 2 Shields, 2 Damage/5 Slowmo otherwise
Core: 20 Bumper/Shield
Form 3
Counter-Orb Curse: 3 Cannon
2 Leftward-Angled Small Guns: 2 Cannon/2 Freeze
2 Rightward-Angled Small Guns: Turret/4 Repel
2 Left/Right Big Guns: Carve/Shield
2 Top/Bottom Big Guns: 4 Barrage/1 Erase
Each Xulgon Minion: Cannon/8 Dash if a player has no Shields, Airburst/2 Slowmo otherwise
Trivia
Medusa, Kraken, Execution, and Phobia are the only official bosses that lack invulnerable pixels on their bodies.
Medusa, Guardian, Execution, Hermit, and Storm are the only official bosses with non-gun parts.
While Xulgon's minions can spawn in the stage, they can only appear with Medusa's third form, and are so uncommon that Hermit is widely considered the de facto introduction to said minions.
Medusa's first and second form's cores have identical spritesheets. There are multiple unused sprites for it.
Since the Redux update, Medusa is the only Normal-difficulty stage in the official campaigns that has the Airburst item -- whose source is obscure nonetheless.
Prior to the Spawner pickup's addition to Xulgon in the Redux update, Medusa was the only main campaign stage to have said powerup.
