Easy lvling, money, and "mini boss" machine parts.
Titled pretty much sums up what it does but here's how it works.
Goto the Barren Temple on your second play through, you might beable to do it sooner (I don't know if you can do it on the first for sure, might beable to after completing the story line quest to find the King of Facade). Should work on any play through after the first for sure though.
After arriving at the Barren Temple, go through all the trials 'til you get to the one that says you are forbidden to use magic. When you get in there, put on words that increase attack, experience, and set up one of your magic buttons to use Dark Blast. Run up to the boss and kill him real fast, but leave his minions alive. Grab his drop, (Complex Machine, Simple Machine, or Elaborate Machine), then push the Dark Blast button and let off so you don't ever hit a point of running low on magic, just long enough for it to shoot off it's first attack. It'll start the trial over and respawn the boss.
I've killed him over 150 times with an average of 10 seconds per fight, and out of those 150 times, he's only not dropped a machine part 3 times. The drops rates for each part seem roughly 33.33% per machine part, I went in there without a single machine part, (I think), and all my Machine parts are roughly the same amount.
Whenever you've got 99 of each machine part, kill the boss and his minions, head back to town, take them all and sell all your spares. You'll need to keep 6 Simple, 8 Complex, and 5 Elaborate Machine parts. The rest will sell for about 1k per spare part. With the 30% exp bonus word on easy, the boss gives me 1040 exp per kill and only takes me two hits with a lvl 3 Phoenix Spear and 20% Attack Power boost.
Just remember, DO NOT DIE, this could cause you to lose all your machine parts, I'm not sure but I'm not going to take a chance and test it. Go in there with a few healing items and if you ever run low, remember his minions will drop Healing Salve here and there. So if you ever start running low on healing salve, kill his 3 minions but leave the boss alive, use magic and farm them for healing items 'til you are satisfied.