Just FYI, finishing Legendary mode does not unlock the Hard mode trophy, but you can cheese it by reloading the last save point and changing it to Hard in the settings.
A couple of tips:
- Make sure to have unlocked Dondoko Beam and spam it as much as necessary during Kasuga's parts
- Same thing for Essence Of The Dragon God during Kiryu's parts
- Always bring a Geodancer with you (Essence of Tropical Tornado and Essence of Natural Healing are both insanely broken) and give them the MP recovery accessory (War God's Talisman)
- If you have the DLC, you can spam Essence of Showboating (Linebacker) to build up armor fast
- Many of the mobs are HP sponges, make sure to use debuffs to make them go down quicker (I personally like spamming Vital Vibration (Night Queen) followed by heavy physical skills)
If you've completed all dungeons during your first playthrough, this shouldn't be too hard. Good luck!