Ragna.... It's just an annoying process to fight him because of his huge and fast hitboxes. 5B works good, but it's just so hard to time it correctly to counter a Ragna 5B... 2D'ing against Dead Spike is good, especially if you can see it coming during a blockstring. I just wish 4D worked more against Ragna's air approaches, but all of his hitboxes are either too big or allow him to counter 4D when he lands... It's just a very frustrating match up X__X.
Besides all of the match up knowledge you need, I think the next big thing is patience... It's not easy being pressured by Ragna, but staying calm and watching what they do is key. Another thing to note is that Ragnas like using Gauntlet Hades if you're being pushed too far during their blockstrings.