i guess i might as well put my 2 cents in
when nirvana whiffs an attack, you can attack her in order to reduce her gauge. i wouldn't suggest doing anything else otherwise, since you leave yourself open for carl to counter your approach. however, if you plan on doing that, make sure you end in either 6A or 3C, since both are jump cancellable. 6A is preferable, as the hitbox is better for if carl tries to airdash in while you're attacking nirvana.
in general, you'll want to respect carl's neutral. brio can become your worst enemy if you're not careful about your space control. also be wary of 4D, as it's perhaps one of carl's better defensive tools in his arsenal.
once you get in carl's space and go offensive, make sure he respects your pressure. the only invincibility carl has lies in his vivace, which is only body, head, and projectile invincible, which means it's susceptible to lows. if you notice carl using vivace a lot to escape your offense, start using 2C.
if you ever find yourself caught in carl's rhapsody, keep in mind that if you IB each hit, you can create space for you to ID through. most carls tend to forget this fact, but just be careful of those who are aware of it.
this matchup is probably in favor of carl (slightly at best). if either character is able to sway their momentum to their advantage, they end up winning. however, ragna's options not being beneficial most of the time against carl, so he has a harder time going in than carl.