Here is the full list of all 58 Final Fantasy IV (3D Remake) achievements. It takes around 80-100 hours to unlock all of the achievements on Windows.
You finished the game on Hard difficulty.
You finished the game.
You obtained all the Onion items.
You obtained Adamant Armor.
You acquired all hidden summons.
You played through all Namingway events.
You traveled across the entire map.
You obtained Ragnarok, Holy Lance, Murasame, and Ribbon.
You earned 500,000 gil.
You obtained Excalibur.
You defeated the CPU.
You defeated the elemental archfiends.
You raised Cecil (Paladin) to level 70.
You acquired all summons.
You defeated 2,000 enemies.
You went to the moon on The Lunar Whale.
You obtained 20 augments.
You defeated Demon Wall.
You defeated Rubicante.
You completed 60% of the Bestiary.
You raised Cecil (Paladin) to level 50.
You talked to everybody in the Training Classroom.
You defeated Dr. Lugae.
You earned 200,000 gil.
You talked to everybody in the Developers' Office.
You defeated Calcabrina.
You defeated Barbariccia.
You defeated the Magus Sisters.
You defeated 1,000 enemies.
You earned 50,000 gil.
You defeated Dark Elf.
You raised Cecil (Paladin) to level 30.
You completed 30% of the Bestiary.
You defeated Cagnazzo.
You raised Cecil (Paladin) to level 20.
You raised Cecil (Paladin) to level 10.
You defeated Scarmiglione.
You defeated 500 enemies.
You raised Cecil (Dark Knight) to level 25.
You were shipwrecked by Leviathan.
You earned 10,000 gil.
You defeated Mom Bomb.
You defeated the Sahagin that attacked at Kaipo.
You raised Cecil (Dark Knight) to level 20.
You traveled to Damcyan Castle.
You completed 10% of the Bestiary.
You raised Cecil (Dark Knight) to level 15.
You defeated 100 enemies.
You obtained one augment.
You acquired one summon.
You traveled to Mist.
You played a new game.
You finished the game twice.
You obtained all augments.
You completed the entire Bestiary.
You defeated Geryon.
You defeated Proto-Babil.
You earned all achievements.