[Zakizo]

If the class EXP thing sounds good, I think I have a good way to go about it. When equipped with two different classes, you get the lower EXP modifier. So if you have Black Knight and Evoker equipped, you get the EXP modifier for Evoker. If in a party, everyone gets the lowest EXP modifier on the team (so if you're grinding with an Evoker, you now have the Evoker modifier since you have an Evoker helping you speed things up).

As an idea (based on my own experience in how well these dominate the grind, but probably would need tweaking because I most likely don't know what I'm talking about), the EXP scaling could be somewhere around:
Evoker: 1.0x
Hexer: 1.0x
Lantern Bearer: 1.2x
Magic Gunner: 1.5x
Verglas: 1.8x
Priest: 1.8x
Kensei: 2.0x
Ghost: 2.0x
Grand Summoner: 2.0x
Demon Hunter: 2.0x
Bonder: 2.2x
Engineer: 2.2x
Void Assassin: 2.5x
Tactician: 2.5x
Arbelist: 2.5x
Black Knight: 2.5x
Monk: 2.5x

Please note: I tried to rank them in terms of time efficiency, not necessarily their power.

Base classes could be left alone, or all be given a slight EXP boost.

Champion dungeons also sound great.