You have to beat every campaign again on hard, of course you can use your savegame with the leveled characters and skills. Using the following skill set will allow you to plough through the game with ease. There only a few tricky parts as the Keep/town defenses and Levv's boss fight in Seratem Defense and Maggni's Orphea Castle Defense.
1. Use these accessories and weapons(of course everything is maxed out)
- Lightning Bolt (found in Levv's Crystal Cave)
- Radiant Sphere (found in Galen' Colunei Canyon)
- Prism Ray (automaticly once Sephia finishes Cavern of Queens)
- Ice Storm (found in Maggni's Forest of the Lost or Quake)
- Virtue (found in Galen's Orphea Castle Gates)
- Magic Empower (found in Sephia's Orphea Castle Dungeon)
- Ethereal (Complete Network mode Escort level 2)
- Quick Magic or Quick Draw or anything else you like
- Galen: Storm Blades
- Sephia: Zephyr
- Zazi: Eternal Fate, since I don't the rest of them usefull for the selected skills
- Levv: More or less all weapons are useful, depends on what you prefer
- Maggni: If you want to use many melee skills with him use Bone Breaker or Berserker. Otherwise Titan of Doom is just fine
2. Once you've beaten all campaigns and the achievement doesn't unlock, check the mission stats on each mission. The score displayed depends on difficulty and it should be easy to find the mission you missed to replay.