I tried it once and got wrecked by flare. What's the best way to survive it?
Few options to deal with his super attacks after the 50% threshold. Flare and Tremor are both cast every third turn after 50%, but if the boss is inflicted with silence or blind, she will cancel the special attack for that round. Blind will cancel tremor, and silence will cancel flare.
Because bosses have diminishing returns on status effects, you don't want to just spam cast them. If you instead only use a status move every 3rd turn (fire flask, cursed card, dance of death, etc) then it should always land, cancelling those super attacks. Just make sure you don't lose track of which turn you're on.
Another option is to bring resist buffs (especially earth resist, but wind helps too), so that you resist the tremor. If flare's the only big attack that turn because you resisted tremor, it shouldn't be too bad.
I had to actually repeat the trial twice more to get the esper mission. After my first kill went pretty well, I went back with a more dps heavy team to try to rush through it again for the esper kill, but ended up getting wiped from a tremor/flare turn in phase 2. Third run I went back to a more conservative, tanky setup, and with the knowledge of the mechanics, I handled p2 very cleanly (way cleaner than my initial kill going in blind), and got the esper mission trivially, no deaths the whole run.