Aside from the Jellys, Skeletons, and Corpses, the only other thing I can think of is Cluckatrice Hunt. The trick is that Cluckatrice, and the 3 Cockatrice, will respawn if you leave the zone with one of them still alive. You'd get 16 LP each trip if you decide to kill Cluckatrice (with Golden Amulet). I think that's a lot slower than the Skeletons in Lhusu, though, because it's pretty tough with low-tier equips. As for looping zones, there's the northern areas of Zertinan, but you need to get past Adrammelech so you can push the boulders and connect the pathways. Giza Plains (dry) has a good 4-zone loop, with quick kills. I forget how many zones it takes for monsters to respawn, I think it was 3 or 4, so that would probably be ideal.
EDIT:
Now that I think about it, with Dustia, it'd take about 20-25 minutes to throw 99 Phoenix Downs, so that's 99x6= 594 LP. It'd wouldn't even take 5 minutes to restock on PDs from Migelo's. That averages out to about 20 LP/minute. I say Dustia is the fastest for LP.
EDIT:
Now that I think about it, with Dustia, it'd take about 20-25 minutes to throw 99 Phoenix Downs, so that's 99x6= 594 LP. It'd wouldn't even take 5 minutes to restock on PDs from Migelo's. That averages out to about 20 LP/minute. I say Dustia is the fastest for LP.