Here is the full list of all 44 Islets achievements.
Fall into the Deadly Pit of Recently Escaped Spirits
Visit the Forgotten Grave's Spirit
Reach an Arrow Strength of 10
Open the Final Gate
Defeat GutGhoul
Defeat FilthPest
Collect all 60 Upgrades
Defeat BoneGolem
Upgrade your Arrow Strength
Reverse the Stone Witch's Curse
Defeat Rogue Bot
Defeat Defender Unit
Upgrade your Airship Again
Reach a Sword Strength of 10
Defeat Swamp Frog
Find both hiding spots in Rainy Plains
Defeat the Stone Witch
Buy Upgrade Markers
Visit Snoots Shop
Defeat the Runaway Plant
Collect 30 Upgrades
Open All 5 Paid Gates
Defeat the Forgotten Grave
Defeat MechaRat
Defeat GrappleBot
Upgrade your Airship
Buy the Shrine Shifter
Beat a Boss With Full Health
Complete the Scavenger Hunt
Defeat the Buried Beast
Upgrade your Health
Defeat the ClockMaker
Defeat the Tomb of 1000 Spirits
Collect 10 Upgrades
Defeat the Sky Pirates
Buy Map Markers
Open a Post Box
Upgrade your Sword Strength
Find a hidden nook
Defeat Grave Crawler
Collect an Upgrade
Complete Boss Rush
Complete Boss Rush Without Dying
Beat Boss Rush on Hard Difficulty