Here is the full list of all 40 Jets'n'Guns 2 achievements.
Completing your first item set.
Collect 100 gift packages in Xmas Mayhem level.
Defeating Techno-Kraken from Water World.
Defeating Necrodroid Titan from Moon Colony.
Find secret Xmas Mayhem level.
Defeating Kong 25 - Panzer Devastator from Pirate Base.
Defeating RX-TDY Jet Fighter from Star City 17.
Save the sledger before the tragic end.