Once you have accepted the side quest Wisdom's Echo, make your way through Aveclyff until you reach the 'Lower Central Region'. There are 20 landworm groups here, easily linkable by timeshift, that are instantly respawnable. Each group of 20 landworms yields 99 enemies - or +99 on Battle Chain (After 400 chains you learn skills twice as fast). Also if the battle lasts 10 rounds or more there is a gain on XP [unverified]. Only leaving for the world map or completing the quest will break the chain.
The door at A4 will lead you to a teleporter should you need to exit to the world map to re-stock - then accept the quest again to repeat.
Engaging hint: Start off using the engage button 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 round. Run in circles and repeat until you get the hang of it and bag all 20 every time.
XBOX: Note the Rush only skill grind trick is more effective for leaders (and can be done anytime) although this here might be useful for training soldiers.
Kadamony's Test
PC Hard Mode just before entering Undelwalt and engaging The Fallen
Start--BR24 , End--BR31 , 3152 Landworms chain, always <10 rounds.
Skill Increase: From near-base skill levels to... Snowfall (Rush), Zeal's Virtue (Gaou), Heaven's Door (Caedmon), Hawkrang (Torgal), Snowblind (Roberto), Cachexia IV (Lukorra, Zolean). On average, wield and weapon skill levels increased beyond rank 16. Use growth Remnant accessories for slackers.
Stat Increase: Total HP almost tripled, 8 players have over 100 Strength, 4 have over 100 Intellect. Many gained over 60 points! Speed grows much slower, most characters at 30. Typically got 4 separate stat bonuses on 70% of characters every battle, and 2 to 3 bonuses on the rest.
Grind Details: Average 15min per battle, 10 hours total. Used Mystic Seal to reduce normal attacks and counters to 0 damage, maximizing skill usage. 20% of the time monsters survived single powerful combat skills. With the added defense, healing was not necessary (to say nothing of Gleny's corpse). BR growth was every couple battles at first, then four or five. 30 to 31 took nearly 10 battles. If you start here at ~BR28 you're still good as far as efficiency is concerned. Characters accrued about 400,000 gold each during the grind. Stopped grind at BR31 after the last weapon art was learned as not to dwarf end game recruit stats (The Seven). Keeping Wisdom Echo open until end game as Jager and Glenys are worthless compared to the grind benefits. Note you would also sacrifice Hundred Flowers to keep this quest open.
Playthrough data and hints: Timing was just before The Fallen battle, unlocking all players except Glenys and Jager. Minimized BR on entire playthrough. Characters always matched final wield style and weapon type, so minimal skill ups were not wasted. Turned off non-combat skills in general except for 4 magic players and a smattering of psionics, as combat arts is the best source of spike damage. Killing all bosses was a breeze with only a few game overs at points where characters and gear were limited. Carried over a 7 million budget from previous playthrough and components. Used player gold to upgrade expensive weapons. Managed to get dozens of Remnant and other weapon upgrades from disassembling all gear on prior playthrough before final boss (Auld weapons are great). Did not have Ancient Ruins access on prior playthrough, but completed all possible guild quests. It is recommended to stock up on magazine drops and the rarest components on your prior playthrough.
The Fallen: At BR31 in PC Hard Mode, The Fallen fell in 3 turns...Torgal capped him in the face with Lugh's for over 200,000 damage before his HP was even blinking. Cachexia nullified all of his attacks, but even without it, heavy use of psionics can keep him at bay with fewer casualties.
The Enlightened Seven: They gave me a hard time with Galaxy, since I had only Rush's Orphic Ward. With the Vampire (magic dodge) formation, and some magic evasion shields, I pulled off a kill on Ludope with only 1 union left using Talisman and Orphic Ward. Heavy use of psionics/morale control is a must.
End Game: At BR43 in PC Hard Mode, just before the final boss chamber, I revisited our critter friends with every recruit except Gleny's and Jager. This BR is after completing all possible quests (other than Wisdom's Echo and subsequent quests) and some moderate farming. This time I took the grind to 1071 chains. BR increased by 1 to BR44. Surprisingly, I got almost no stat up bonuses. Each fight I got 2 to 4 increases out of all 18 characters, primarily HP and AP. Skill ups were normal, as those are not dependent on BR. Don't bother keeping this quest open when it's time to beat The Fallen, unless you want to train The Seven without affecting your BR. Of course, you could always fight any boss type enemy with Mystic Seal formation.
Conclusion The sweet spot for the PC Ultimate Grind (at least in PC Hard Mode) is BR26 to somewhere between BR32 and BR40. PC Ultimate grind is well worth the while and with proper planning and execution, you will break the game too!
Player Test
Tested at BR54 and reached 1,944 battle chain count before being moved to BR55. Each active member, on average, gained 4 upgraded/new arts. They also gained AP/HP etc every battle. Each linked 20 landworms battle lasted over 10 rounds.