So the fastest way to farm xp by soley boxing, IMO, is while leveling to run rifts. I generally start at 2 notches above the lowest difficulty, and slowly kick it up. I know its time to raise the difficulty when I start one shotting general mobs en mass. for paragon leveling I farm GR keys and push.

The fastest way to level though is to get someone to powerlevel one toon to max, and then farm up the chapters for the gear set, and then you bring in your slaves and park them at the entrance to a rift and you run through and kill everything on t6, and just teleport them to you as you progress through the levels. Once you get a decent geared toon you can boost slaves in a few rift runs to max level.

I love boxing wizards, necros, and WD's.