There are quite a few openings in Ragna's blockstrings.
Whenever he dash-cancels 6C or 5D, he loses advantage. If he's using 2B to reset his pressure, he's heavily minus on block. 2B is -7? I think.
If the Ragna you're fighting against likes to IAD after 3C or 6C in pressure, IB and dash forward. You'll run under him and can hit him with 2C.
If he likes jumping in with j.C (haven't tested for j.D) then 2A him. Litchi will go under j.C.
Whenever he goes for 6B (the overhead), you can hit him with 5A or Tsubame every single time.