Ashe USB more or less solo-ed this one with R4 Voltech (hah). Ran double healers with Larsa + Panelo, while Vaan did some debuff stuff and thief's raid (probably more damage using an element skill due to weakness) but was mostly inconsequential apart from self healing so healers could cure others.
Larsa: Shellga/Curada/BSB - Legend Dive (w cast White).
Balthier: Ruby Spark/BSB. Just DPS, could have anything here, such as Ashe.
Magicite: Mom Bomb contributes to realm chain and was hitting damage cap for me, the buff helps Balthier's fire DPS and possibly Reks if he uses fire too. Also had the dampen dark one inherited with dampen dark and dark resist accessories, the promised Incubus Darkja rape was pretty tame through that.
On the D240 I skipped the damage immune phase, that's probably something to aim for to sub 30 the D280.
My second pull was Basch USB which helped me get the sub 30. Not done the hardest difficulty yet due to not having much URL time but I got 45% on my first try so it's doable for me to get the 50% which I'm happy with for now.
Key to sub 30 on the middle difficulty is casting realm chain on turn one and having some of your dps also start on turn 1, this has been true for all three for me.
I also managed to skip BOTH immune phases by unloading SBs at 50%, this was Ashe OSB under enlightning from her LMR and Basch USB medium boostga, Balthier BSB and Vaan BSB - so all old DPS stuff.
Multihit abilities are also really important, Voltech was capping from the start without a 4* dive and Burning Snipe(machinist fire 6 hit)/Soul Fire(thief fire 4 hit) crept towards capping with realm chain and Balthier Legend Dive proccing imperil fire. Basch had no fire dps so he just did Assault Strike (5 hit knight) which was fine.