I know it's a bit early but I had a chance to play against some decent Platinum players both in arcades and on PSP Adhoc Party matches. So far I think this match-up might be one of the more difficult ones for Rachel.
From experience, Platinum feels like I'm fighting a cross between Ragna and Rachel. With bombs or missiles she has tools to disrupt your own zoning or summoning attempts. With hammer or frying pan she gets access to more damaging combos. She also has normals that out range Rachel's.
Against bombs or missiles it's best to stay on the defensive. Try jumping over missiles and misdirecting bomb shots. If you block missiles or bombs it means Platinum gets in (bad). One way of shifting momentum against missiles is to space yourself correctly so you can jump and attack to get inside after successfully jumping over a missile. Use wind if you have to. Bombs you can just dash in and wind if you can predict when Platinum will launch some bombs.
Platinum up-close has decent pressure. One frame trap involves her 6C attack that's cancelled into Swallow Moon that is immediately cancelled into j.C. Platinum does have a command grab which closes distance since she has to run towards you first. Landing this nets her a "reload" of the current item she has equipped or if she has no item, she gets the next item queued equipped. If Platinum uses this grab mid-pressure you can avoid this by jumping if you think they will commit.
Your gameplan should generally be the same. Corner carry into corner trap. Do not play this match-up midscreen.