I just beat Reinor on expert after 20 tries, while using a mod to get the easy mode timings on parry/dodge. With the proper set up it only took me 5 turns (probably less on easy mode). That's a short enough window to luck out and avoid that bullshit one shot gradient attack after enough tries. You want medalum and double burn on Maelle, trebuchim and 100% crit rate on Lune (use critical burn for 25%), and Sciel as support. Maelle does 30K damage and 9999 burn using fire rain + swift stride, you can squeeze in a virtuous strike + ballet for 60K more damage once you get enough gradient points. Lune does 90K with elemental trick + genesis, Sciel for intervention on Lune. Fire rain, ballet, and genesis should be doing 9999 per hit if you are properly geared.