The highest EXP you can get when your 90+ is metatron(earthrift) he is preety easy to
defeat he's most powerful attack is spellcanon which you need to block or you'll die the
exp you get from him is 5000 or 4000...this is the fastest way i can think of

The highest exp i got is from dela rubesi you need to be level 60 up to defeat the boss
there onba... You will have 30,000 exp in your first 3 characters that battles... And half
of the last three groupmates of yours