Once you have accepted the quest Wisdom's Echo, make your way through Aveclyff until you reach the very bottom of the Lower Central Region. There are 20 Landworm groups here, easily linked by Timeshift, that are instantly respawnable. Each group of 20 Landworms yields 99 enemies - or +99 on Battle Chain.
After 370 chains you earn twice as much EXP, and only leaving for the world map or completing the quest will break the chain. Hence, you gain a massive amount of Stats increases, new Arts, and Art upgrades by defeating very easy enemies over and over.
The door at A4 will lead you to a Transporter should you need to exit to the world map to re-stock healing items - then accept the quest again to repeat.
Engaging hint: Start off using Timeshift to aggro a few critters. Monsters will lose aggro if you Timeshift and do not tag them again. So use Timeshift to position yourself rather than tag (and both when you get good). When Timeshift wears off, be in a position where you can aggro a new group with the engage button as well as the old group that was following you from a previous turn. Run in circles and repeat until you get the hang of it and bag all 20 every time, although engaging at least 10 or so still provides great success.
XBOX: Note the XBOX-only Rush only skill grind trick is more effective for Leaders (and can be done anytime), although this Landworm chaining might be useful for training Soldiers.
What You Must Temporarily Sacrifice
Note: In order to utilize this grinding spot, the following Leaders, loot, and locations must be temporarily sacrificed since you cannot complete the quest. When you are finished grinding, the quest line can be continued and the various rewards collected.
Locations: Areas of Aveclyff unlocked by later quests, Flaumello Tower, and the Ring of the Labyrinth Guild in Athlum.
Please visit Bai Ze's Talk page for an analysis of PC Ultimate Grind vs Bai Ze grinding.
Videos of PC Ultimate grind
Getting 20 Links in one single pull
PC Ultimate Grind with all skill ups and stat ups with English commentary