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