By exploiting relay unbuffering, enemy knockdown, Speed Aura Lv. 3, and a minimal deck, relay combos can go way past the intended maximum of 25 cards. The point of this video isn't to reveal the longest possible combo, but to demonstrate and explain the strategies. It is easy to do longer combos than this, but I didn't want the video to be any longer than it already was going to be. (And it just so happens the game would have softlocked before the victory pose had I just gotten one more card in, so I'm good.)
Relay unbuffering: If you press X after playing a relay weak attack, characters that are already done with their magnus selection will get in the action bar immediately. This, along with Speed Aura Lv. 3, lets their attacks and cooldown end soon enough to keep the combo going, even after 3 or more actions.
Knockdown: Relay unbuffering will only work as long as no enemy gets in the action bar. Fortunately, weak enemies get knocked down easily and won't get any chance to recover during the combo. But even if the enemy is too strong to be knocked down constantly, there is a workaround...*
Speed Aura Lv. 3: This gives you a 100% turnover speed bonus so the delay gauge is cleared twice as fast. Auras disappear after 10 minutes of battle time, but you can also use Pegasus Anklet for an unlimited 80% speed bonus, which should make an endless relay combo possible.
Minimal deck: My deck only has 2 weak attacks, 2 medium attacks, and 2 strong attacks. The benefits are obvious - every time I prepare a turn, the cards from two turns prior are shuffled back into my deck, usually in the very same order I played them (except for the weapons which I only need later anyway).
Since TP bonus = (# of cards - 2) ^ 3, I gained 1,860,867 TP from that combo. Also, judging by my shortest and longest battle times that were both reset to 0:00, extremely long combos could potentially corrupt save file data. Be sure to not overwrite a playthrough you intend to keep.
*Migraine Mirror: Using this slows down all enemies. The slowdown is derived from this formula: cooldown = regular cooldown / (1 - 0.2 * # of mirror uses). While it seems that 5 uses are enough to make enemies permanently unable to attack, this only holds true if they don't act twice between the first and fifth use.
Temporary stat changes follow the two-turn principle. Migraine Mirror, for instance, decreases two speed boost values by 0.2. The first value determines the momentary slowdown of the enemy, while the second one is the slowdown after the next turn. If the enemy manages to act, the first value is overwritten by the second value, and then the second value is reset to 0.
This means you have to keep using Migraine Mirror fast enough to be able to reach a speed boost of -1. (A mattress can help, but it may clutter your hand even more.) You can open the target menu to check the enemy's delay gauge - if it's frozen or fills up even more, the enemy can no longer attack.