D-Spike is really easy to punish. If the Ragna is spamming it constantly and is always following it up with the upper punch, you can punish it with 3C after guarding and then setup a zoning game or follow it up with 234D into RC.
Alternatively, if you 3C before the second punch and they end up close to you, you can follow it with 214A, then combo from that.
One more decent way to punish it is to simply dash towards them and grab. Mix-up between your punishes though, or else they'll start countering them.
Remember that his D-Spike isn't like Jin's Ice Car; it won't "hop" over a 5DD if you're not fast enough. You can still cancel Ragna out with 5DD if you see a ground rush coming.
When you're zoning him from afar, don't forget about Arc Pulsar. You can always keep them guessing if you use it if 5DD is getting old. You can get close up to him this way, and it's usually in your favor unless they predict it.
IAD's are easily punishable as well, but in most cases, it depends on prediction. As you keep playing, you'll get really good at predicting IADs, and you will be able to punish them well (almost instinctively, to be precise). You can also dash under IADs; it's pretty fun to do that and zone them once they land.
one more thing; when you're being pressured, use 3C as a poke if you're pushing them back/see a gap in their offense, and use 5A as a poke if they're staying close to you while pressuring. Use barrier guard to force them into using a Death Spike to continue their pressure if you want, then punish them afterwards.
