There are three items in this game that will allow you to gain levels at an amazing rate.
Learning Ring:Increases EXP by 30%.
Coin of Fortune:Increases EXP by 50%.
Book of Refining Gold:Turns each Monster into at least 10 Mystic Gems.
So, from this you can see that if you have a character equipped with 2 Learning Rings and a Coin of Fortune they will be getting roughly double the rate of experience that everyone else is. This will allow you to get a Character like Valkyrie to the high levels up fast.
Hidden Items
Some characters in the game once obtained have special items. some are significant like Arngrim's Dragon Slayer. Some are as useless as Lleweyn's goddess pendant. Here is a list on some that you can obtain:
Arngrim- Return to his bedroom to obtain the Dragon Slayer
Janus- Return to his home to receive the raven slayer
Belenus- Enter Asaka's bedroom to obtain the crushed flower
Mystina- enter her room to get the Infinity rod
Kashell- to obtain his sword return to the Camille cemetary.
Badrach- talk to the little girl in the Camille church to obtain the woven bandana
Yumei- return to the beach at Hai Lan to get the cerulean lapis
Nanami- enter the Dragon God's palace and return to the shrine she met Minayo at to receive the DragonBane sword
Lleweyn- Return to the forest where he last hugged his girlfriend to recieve the Goddess pendant.
Stronger weapons
This only works in normal and Hard mode. As soon as you access the stage "The Tower of Lezard Valeth" You will be able to find the creation jewel. When eqquiped on Valkyrie, the broken weapons and armor can now be transmuted into the powerful Glance weapons and armor. Top notch gear until you hit Seraphic Gate. It will also make the tome of Everlasting life into an ice sword, capable of obliterating all fire based opponents in one hit.
Unlimited Experience Points
When you get to the arkadian ruins you will come across a room that traps you inside until you defeat the Brackish Muck creatures and the Rib Foragers/Necrophiliacs. After the battle you get 10,000 event XP. The door opens. You may then leave and turn right around again to have the whole scene replay itself. You will only get the event XP once. I am doing this on normal mode. At this point you can do the "Switch the Emeralds/Zoe Bracelet to the character about to lvl up manuever". Just enter battle and have your magic-user blast with the Mystic Cross!!! You will often get Exp Crystals and treasure chests. Around 10,000 XP upon each entry to the room. You can max all your characters out.