Here is the full list of all 35 Shotgun Legend achievements.
You have found all the crosshow upgrades.
You have found all the dynamite upgrades.
You have found all the wave cannon upgrades.
You have found a way off of Loriam, but where will you end up next?
You have found all 40 monster cards.
You have found a dynamite upgrade. Collect up to 9.
You have found all the rim fragments
You have found the alien map.
You have found all the health upgrades.
You have found the amplifier.
You have found 30 monster cards.
You have found the gravity ball.
You have found the gas mask.
You have found all the shotgun upgrades.
You have found another shotgun! I guess he was out of swords.
You have found the devastator attachment.
You have found the crossbow.
You have found a crossbow upgrade. Collect up to 30.
You have found the cooler.
You have found the banjo.
You have found the barrier generator.
You have found a shotgun upgrade. Collect up to 16.
You have found 20 monster cards.
You have found the raft.
You have found the wave cannon.
You have found the candle.
You have found 10 monster cards.
You have found the dynamite.
You have found a wave cannon upgrade. Collect up to 9
You have found a piece of the rim.
You have found a health upgrade. Collect up to 16
You have found a monster card.
You have found the compass.
You have found the doppelganger.
You have found the shotgun! Well, I guess you already had it.