Honestly, I found fomoire to be better than the moblins if you're leveled high enough to take him down. The moblins have a chance to drop more than one crystal, but fomoire is pretty much guaranteed to give you one every time. And since you only need a few crystals for leveling up it doesn't take as long with fomoire unless you get extremely lucky with the drop rates off the moblins.
Then again I've had really bad luck with this game. It took over 4000 turns of the slots to get 4 jackpots and finally the fragment. So in my case guaranteed drop of one > chance of multiple simply because the randomness makes it never happen for me.
"So how were those noodles?" "A little spicy." "I love you, Grunt". "Heh heh heh"
Yeah, farming Moblins for Power Crystals is definitely a shot in the dark every time you try. Sometimes you can get two drops of 3 crystals each two battles in a row and sometimes you won't get a drop for 20 battles. I still much prefer it to farming Fomoire, though, as Moblin battles are laughably short.
No, I killed the Major Moblins first. I'll try killing the Moblins first.
Though recently I've just decided to farm Formoire or whatever. Got about 20 but he takes a bit longer to beat and he only gives one.
UPDATE: Surprisingly, this tactic has gotten me 9 power crystals in 20 minutes.
Yup, killing the Major Moblin first allows the Moblins to Rise to Power, and if the do, they won't drop Crystals. So, kill the Moblins first, so you get 3 Moblin kills -> chance for 3, 6 or 9 Crystals.
-- Your Guide to Final Fantasy -- http://www.youtube.com/user/soigge