In the post-game, there should be a rumor through Seraphie titled Challenge From a God.
Inside the Coffee Shop, Enna Kros will be awaiting your challenge.
I would recommend being above 80 before taking her on, bring a lot of Hi-Ethers and don't use mirages weak to the light element.
It's not particularly difficult as there's a sort of a pattern, it's just long really.
I will give you damage estimates but these are based on level 96 average characters.
Enna Kros is accompanied by Tama and Seraphie.
Tama and Seraphie sport 48k health roughly, while Enna Kros about 52k.
Seraphie primarily uses 3 abilities: Loud Whisperwind does a small amount damage to all party members roughly 1.5 to 2k.
Rumor Radar: Points out a weaknesss of yours for Enna Kros to do a Telegraphed attack.
Tickle: 2k damage to one person but doesn't occur very often.
Enna Kros I believe has at least 7 abilities (possibly more).
Her basic attack, she blows a kiss at one of your party members which dealt 2k in damage.
Quake which I resisted but still dealt 2k.
Kind Of Holy: Hits both party members twice. With no resistance it dealt about 5k damage.
Waterja/Fireja/Thundaja/Blizzaja (Maybe other elements too): Whenever Seraphie uses Rumor Radar, Enna will immediately follow up with that a -ja spell on whoever Seraphie identified. Big 7k+ hit at least.
She has some sort of reflect as well, your spells will damage her but you may get damaged back.
Uses Haste, Regen, Faith, Foxfire, Dispelga and Timewalk.
Foxfire dealt around 2-3k damage.
The real annoying ability is Timewalk, basically whenever you deplete her own, or the other two's HP. She will auto-cast Timewalk to restore them to full HP. I didn't count how many times she does this but I'm assuming 9 times.
It's really just a war of attrition but I'd recommend targeting Seraphie until Tama is out of Timewalks as she's weak to ice.
Just pummel her with Blizzara until Tama runs out of revives while keeping your party HP up.
I would suggest using Physical abilities on Enna Kros (Meteor hits hard).
It's basically a repeat of movesets from them.
Tama boosts/foxfires you.
Enna Kros will Quake, or basic attack, using Kind of Holy every so often, and -Ja spell after Rumor Radar.
Seraphie will basically use Whisperwind but every so often Rumor Radar.
Once you win, you'll get an achievement some mega-ethers or w.e and on the chair next to her will be a shiny Initial Regen Mirajewel.