Best EXP Farming Location - Wild Arms 3
The best EXP Farming Location in the game is one that only becomes available much later on during Chapter 3. After you start the Telepath Tower Side Quest and destroy 5 of the towers across the world you'll cause Creeping Chaos enemies to start spawning (pictured above). These enemies will give you 10,000 EXP and 10,000 Gella for each one that you defeat.
Additionally, in Wild Arms 3 each time one of your characters is afflicted with a Status Ailment they'll gain a bonus .1x EXP after a battle. Creeping Chaos enemies have a few abilities most notable is the Arc-en-Ciel ability which inflicts 7 Status Ailments on your entire party. This means you'll get a bonus .7x EXP each time they use this skill.
Creeping Chaos enemies have a few more abilities that inflict status ailments such as Human Experimentation and Direct Voice as well. In fact all of these skills are the reason they're such a great enemy to farm for EXP because you can get 9.9x bonus EXP after each battle very easily. This means that the 10k EXP you'd normally get will become 100,000 EXP.
If you fight 3 Creeping Chaos enemies at one time and stack the EXP bonus rate as high as it can go - then instead of 30,000 EXP you'll now get nearly 300,000 EXP. Using this trick you'll be able to get max level in just one day of playing. To see a video of how this all works check out my Wild Arms 3 - Best EXP Farming Location In Game Youtube Video.
You can fight these enemies wherever you'd like - I personally grind outside of Baskar Colony since that is a low level area. Any fights you get into outside of Baskar Colony with a white exclamation point you can guarantee will be a Creeping Chaos since everything else is too low level.
With all of that information out of the way - let's talk about the Status Ailments. Without proper protections on your Mediums you will have a really hard time during these battles due to how often the Creeping Chaos enemies use various ailments. To resist these ailments you'll need to collect various items and 'Equip' them onto your Mediums then assign them as Personal Skills to your characters.
Below is a list of all the ailments that these enemies inflict and where you can find the items that will allow you to resist them.
Moonstone (Poison): Drops from Cockatrice in Yggdrasil; Crab Bubbler in Fortune Gear area; Dust Man in Fila del Fia area; Gerbug in The Unclean Mark; Manticore in Infinitum & surrounding area; Moss Fungus in Humphrey's Peak area and more locations.
Clear Chime (Confusion): Drops from Myconid in Ruins of Dreams
Hazel Sprig (Disease): Drops from Axe Beak in Little Twister area; Carrion Crawler in Abyss Area; Gagison in Ka Dingel
Heart Leaf (Misery): Drops from Elbucky in Laxisland area; Saracenian in The Secret Garden Area
Blue Bracer (Paralysis): Drops from Black Buzzard in Dim Root Path area; Cursed Corpse in Leyline Observatory/Little Rock area; Drill Mandrill in The World's Footprint area; Eel Volk in Sacrificial Altar (and surrounding area); Oak in Lunatic Garden & Little Twister Area; Sandcrab in Survey Point area; Walking Dead in Fila del Fia
Alarm Clock (Sleep): Drops from Mushussu in Nidhogg Pass; Twin Tail in Humphrey's Peak area
Memo Pen (Amnesia): Drops from Dryads in the Westwood Area; Rock Busters in the area North of Boot Hill; Shriekers around Yggdrasil
One piece of advice I can give you is to unlock the Black Market so that you can purchase an unlimited amount of the aforementioned items. When you visit the Black Market and sell any of the items mentioned above the shop will start selling that item for a premium. Essentially what this means is you only need to farm 1 of each item mentioned above, sell it at the Black Market, then you can buy the rest of them.
If you need help finding the Black Pass which will give you access to the Black Market check out my Where to get the Black Pass Youtube Video.