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