FunkyP is right, if you IB the first hit of Hell's Fang, you can punish it, if you don't, you are put in a little game of Rock-Paper-Scissors with Ragna.
Block-Grab-Jab
Alot of Ragnas like to Inferno Divider when you block HF, this will beat anything you try to do, except blocking, block if you think they will try this, and punish with a heavy combo.
If a Ragna thinks you are gonna keep blocking, out of fear that you will be hit with ID, they will attempt to grab you, if they do this, you can beat them out with a quick jab, or your own grab.
Some Ragna's aren't as risky, and will attempt to block after you block HF, if they do this, you can grab them.
If the Ragna is "Packing Heat"(lol) beware, because they will rapid cancel and continue pressure.
Your best bet overall is to block, break the grab if they try it, and be patient, wait for them to do something stupid and unsafe, then punish them for it.