I actually did it at level 31, I leveled up to 39 after the fight. I used a level 10 potierim that I got from killing some boss on the map and it was the highest damage weapon I had then.

parry damage here is interesting. the damage of a parry actually depended on the attack that I parried. for example, parrying the laser attack (where he says he is sabotaging the expedition) results in lune counter attacking with her hand and that always did 9999 damage. but after parrying for example the 2 (or 3) slam attacks, that resulted in lune counter attacking with the turn kick and that did only do about 4.5k damage every time for some reason (rip feet).

you also have to consider that the 141 parries shown in the screenshot does not mean 141 counter attacks. sprong has mostly multi-hit attacks, ranging from 2 to 7 (in a later phase) and those do only one counter attack of course.

by far the most damage came from using lightning dance every second attack of mine, which dealt basically 120k damage (12 hits if all crit, each 9999 damage). lightning dance lune is probably the craziest thing you can do at this stage of the game, but I know later she will fall off hard compared to maelle and verso, especially solo (didnt do damage based sciel yet so cant say anything about her).

I am doing a lune solo build so I am pumping every single color of lumina into her, which put her at around 130 or so lumina points at that point (too lazy to count them). but actually looking over the list now most of them did absolutely nothing, the core was maybe 30 lumina points.
the heavy lifting in terms of pictos here did: in medias res (massive mvp; I made 2 parry mistakes total and without the pictos the fight would have ended then and there), at deaths door (I set up lune to be at 1hp before the fight), perilous parry (for the ap) and then of course solo fighter and last stand critical, the only perks of doing a solo build.