I just leveled my BLM most of the way 66-75 (minus probably about 3 levels from random tiger PTs) in campaign. 70+ the best sub is SCH hands down. Make sure you have RR up always.
I find that if I went all out nuking, my exp suffered pretty badly. I think the magic damage category cap might be kinda low.
Use Enfeebles! Blind, Stun, Sleep. Casting 40 enfeebles to cap enfeebling exp. "No effects" do not count towards the total but resists do.
Melee! Get a decent staff skill, 215 at least for spirit taker. When you're low on mp, don't rest (unless you have Aresian grip
), put on some accuracy, ride drain and aspir and use spirit taker. The category cap on Melee damage is either super high or there isn't one.
Appropriate use of -GA spells. See a bunch of units summoning a turret? Parsimony + Ga their asses. It's cheap and gets you a slew of EXP in one go.
Heal yourself and others! There are separate caps for healing yourself and others (about 8k hp for each). Try to hit both but keep track so you don't exceed them by much.
Pull hate! You get EXP for both taking damage and healing yourself, enough said. Try to pull enough hate to get smacked around but not enough to get killed.
Here is a handy reference guide
. It's not perfect but it's close enough.