I just went against a Litchi today and my best asset was patience and a keen eye. Staying in 5B range is critical for Ragna, but even still, I made liberal use of the barrier guard - this forced the aggressive Litchi to restart their offensive which gave me more opportunities to catch a mistake. I punished toss aerials with 6a until the flow changed. When it did, I did my best to bait the DP then rush down after she didn't have the stick and I used my gauge to RC and continue pressure.
Her gimmicky combo (which all consists of the same move, I swear), has to be stopped before it starts. The best way I did it was with light use of IAD to j.C to j.D.
Turtling, mixed with some pokes to provoke aggressive, is really the best way to go about this match up.