The spawn rate is WAY higher around this yellow cactaur statue. I can say this from experience having switched to this area after farming the marsh-area for 2 hours. Instead of a 30-40% I'm getting a 80% spawn rate.
Dont know how you get 80% spawnrate of Moblins around the yellow cactuar stone. I was running there for over 1 hour and encountered a handful of moblins. And the direction "northwest where you throw mog" ... NW of the yellow stone i dont see any treasureboxes to throw mog to. Maybe the one over the chasm to the marsh ? But spawnrate in that area was awful too. *sigh*
I managed to get a very high rate of Moblin spawns in that area. Slightly north of the yellow Cactuar stone there is a bridge over one of the big trenches in the ground. Running along that bridge I encountered Moblins probably 8/9 out of 10 encounters.
One thing to watch out for is when you start the battle it will automatically target the Major Moblin. You want to save him until last. If you kill him first, one of the Moblins will turn into a Major Moblin. If you kill the three Moblins first you have a chance of getting up to 9 crystals per battle (although that's extremely unlikely).