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 the final threat and completed the main story.
Defeated the Titan and completed the main story on Ouranos Island.
Viewed all Side Stories.
Viewed all Side Stories on Ouranos Island.
Performed Phantom Rush 50 times, outside of the training simulator.
Repaired all Portals on Ouranos Island.
Completed all Challenges on Ouranos Island and fully revealed the map.
Defeated all guardian types.
Defeated all enemy types.
Raised Ring Level to MAX.
Raised Defense Level to MAX.
Raised Power Level to MAX.
Raised Speed Level to MAX.
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.
Defeated the Titan and completed the main story on Ares Island.
Viewed all Side Stories 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.
Viewed all Side Stories on Kronos Island.
Completed all Challenges on Kronos Island and fully revealed the map.
Repaired all Portals on Kronos Island.
Earned 100 Purple Coins.
Encountered a starfall for the first time.
Spoke with Hermit Koco for the first time.
Landed all catches at one fishing spot.
Completed the fishing tutorial.
Spoke with Elder Koco for the first time.
Destroyed 100 breakable objects.
Completed the Kronos Island tutorials.
Performed a Phantom Rush for the first time, outside of the training simulator.
Activated a Power Boost for the first time.
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.