The fastest way by far is to make slaves--you're necromancer! If you have a blessed amulet of balance, you can limit the effects on your alignment, I guess, but you're much better off just going chaotic for awhile. At low levels, you can switch back easily enough later.
I don't think it matters that much whether you use big spells or small spells repeatedly, it's whatever drains your PP out. Bookcasting is nice because it drains your PP by a lot and doesn't use up castings.
But seriously. Use slaves. You will gain Mana 10x as fast as repeated castings.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.