Going by ancient memory here, but the magician manual is a powerful tool if you just spam casting with it of the correct element, have Leander a good deal of power and he rips trough targets, and spam mana restoratives to prolong the spams.
Can't remember which slog is the one you are in, but this strat sadly doesn't work on 27 where you need a strong melee weapon of fire element to do in decent times
I never did level farming, it is doable if you have weapon luck. (even previous DLC can give you weapons that can be used for the entire slog)