I just acquired the Light Rod this morning from this dungeon!
For both the first and second rounds, I got 8/9 possible medals, falling short on the amount of damage taken, but still achieving Champion ranking.
My party to get the Mastery reward consisted of Cloud Lv. 27, Tifa Lv. 23, Rydia Lv. 26, Tyro Lv. 26 and Black Mage Lv.14
In a way, it seems kind of funny to me, I consider you quite ahead as I don't have Sephiroth yet and intended to get the Light Rod to help obtain Sephiroth haha

Anyway, equipment: managed to pull a Crystal Sword 4* Relic for Cloud, combined a few Sonic Knuckles for Tifa, Whip for Rydia, levelled out to 5/5, a staff for Tyro and a Mythril Dagger for BLM.
Abilities: thunder strike and blade bash on Cloud, blade bash on Tifa, Thundara and thunder on Rydia, Cure and Dark on Tyro and Cure (honed to 8 uses) and Poison on BLM.
At this point for me, Tifa and Cloud alone have been able to deal massive blows that kill all the regular enemies in 1-2 blows, so I pretty much used those two alone, skipping turns for the rest of the party without using up any of my other abilities or soul breaks until getting to the Octomammoth Boss, except for the occasional cure equipped to both Tyro and BLM.
After finally getting to Octomammoth, I mainly dealt thunderstrike from Cloud and Thundara and Thunder from Rydia, which did the most of damage. Also landed poison from BLM and Dark (Blind) from Tyro right off the bat. Landing poison might take a few times but is incredibly useful and it frequently hits 397 damage, and Dark helps mitigate the incoming attacks. Blade Bash gave me a bit of an extra kick for both Cloud and Tifa. Once I had exhausted all of my remaining abilities and Soul breaks, I would just let Cloud and Tifa hit him with melee attacks. If one of your more powerful members gets KO'd before they get to cast all their abilities or do a decent amount of damage, especially Rydia, then use the save and reload method! That is, quit/close the app immediately, reopen it and it will take you back to the beginning of the boss round, with all your party members still active. It took me 3 attempts using the save and load method, but definitely worth it! I just kept falling short without having BLM in my party, so the Darkbolt soul break definitely made a difference in obtaining mastery! Before using BLM, I was getting 5/9 medals for the regular target conditions and 3/3 for the lighting vulnerability, but it was never enough to master the dungeon without getting the extra 3/3 medals for the Dark vulnerability!
If you haven't already got your light rod by the time this is posted, I hope it helps!