Here is the full list of all 40 Sonic Frontiers achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Defeated all guardian types.
Viewed all Side Stories on Ouranos Island.
Viewed all Side Stories.
Defeated the final threat and completed the main story.
Defeated the Titan and completed the main story on Ouranos Island.
Performed Phantom Rush 50 times, outside of the training simulator.
Raised Ring Level to MAX.
Raised Speed Level to MAX.
Raised Defense Level to MAX.
Completed all Challenges on Ouranos Island and fully revealed the map.
Repaired all Portals on Ouranos Island.
Defeated all enemy types.
Defeated the Titan and completed the main story on Chaos Island.
Viewed all Side Stories on Chaos Island.
Completed all Challenges on Chaos Island and fully revealed the map.
Repaired all Portals on Chaos Island.
Unlocked all Skills.
Raised Power Level to MAX.
Viewed all Side Stories on Ares Island.
Defeated the Titan and completed the main story on Ares Island.
Repaired all Portals on Ares Island.
Completed all Challenges on Ares Island and fully revealed the map.
Gathered 200 Koco.
Defeated the Titan and completed the main story on Kronos Island.
Landed all catches at one fishing spot.
Repaired all Portals on Kronos Island.
Viewed all Side Stories on Kronos Island.
Completed all Challenges on Kronos Island and fully revealed the map.
Earned 100 Purple Coins.
Encountered a starfall for the first time.
Completed the fishing tutorial.
Spoke with Hermit Koco for the first time.
Destroyed 100 breakable objects.
Spoke with Elder Koco for the first time.
Activated a Power Boost for the first time.
Performed a Phantom Rush for the first time, outside of the training simulator.
Completed the Kronos Island tutorials.
Defeated a guardian for the first time.
Completed all Missions in one Cyber Space area.
Achieved Rank S time, for the first time in a Cyber Space area.