Despite their cute, lamb-like appearence, THEY WILL HURT YOU. They make mincemeat of your party, by nuking them with Meteor (often several times in a row, or blow a character out of the battle with Sneeze), so I wouldn’t reccomend facing them until you’re at a very high level. (You can’t reach it without the Invincible or a Gold Chocobo either I’m afraid, as there is no grass to be found on the island, so the Hilda Garde 3 can’t land there).
For an area for level building with a more moderate challenge, try the ledge above Gizmaluke’s Grotto (which you can reach either by airship, a decent level chocobo or ladder from the Grotto underground). The only enemy that inhabits this ledge is the Grand Dragon. Although a pretty tough enemy early in the game, after a few rounds with them, they’re quite easy (it’s good for raising Freya’s ‘Dragon Crest’ ability too), however, make sure you equip yourself with immunity against lightning, as it’ll make battles against them much easier.
The plateau outside of Daguerreo is also good for this, as Grand Dragons live there too (as well as the occasional wimpy Gimme Cat.).
And you’ll occasionally run into real easy monsters and Worm Hydras above Gizamaluke’s Grotto.
But yeah, those are the best places to go.
If you are at a very low level and are right at the start of the game (the first time you visit Lindblum), get Zidane on his own and head to Hunters Gate. Outside are two forests that contain Bombs. They can be tricky, but will reward you with 178EXP each, which is a fairly high amount at that stage of the game. 🙂
Happy Killing
Muppet
For a harder challenge, go to the island that Enkidoh was talking about, and fight with YANS. (Not Vans)
Those were the only two places I really cared to level up that, so good luck.
A great way to level up ONE player is to kill off the other 3 or make the other 3 get Virus (from Malboros and Chimeras) so they can still fight but not get EXP.