Lie down and take the hit, or stand straight up (don't tech/ukemi in any direction), and jump over the explosion. If it's the faster, blockable one, stand straight up and block low, or take the hit if you're afraid of messing up. Against Natsu, you're going to have to take a lot of extra hits on the ground if you want to survive. The Natsu match-up is actually really simple once you know her juice. Step or Quick Step, then immediately crouch block whenever she goes into PO. Stepping avoids PO B and Hover B, and the following crouch block avoids Hover A. Eat the PO B+K (the low) and PO A. They don't hurt. After any knockdown where she is a character space away from you, never tech roll/ukemi. Either stay down and take the bomb hit (If it's the quick bomb), or stand straight up and block low. If it's the quick bomb, you'll block it. If it's the unblockable bomb, you still have enough time to jump it. Just don't mentally commit to holding 2 and G. Always be ready to shift from 2 to 8 or 7 (or 9B_K, if you're bold) as soon as you see the flames indicating an unblockable. At mid and long range, use lots of a:g:A, BB, and 3AA. Even if she ducks the a:g:A at long range, she can't do anything about it. She will default to A+B or 66B or 66B(BE) at mid range. a:g:A will beat A+B in speed, but 66B will beat a:g:A due to tech crouch. Block if you expect it. 66B(BE) isn't scary as people think. The 2nd hit is a high. Block the first hit, duck the 2nd, take your WR [B] > NSS b:A punish and keep it moving. Try to guard break her by using a lot of BBs at mid range. It's safe, and if you notice her trying to retaliate after your BBs, use A+B after a blocked BB to auto-GI and shut that shit down. Don't pursue after A+B knockdown. Move back and play the zoning game. Watch the rage ensue.
Natsu vs NM is actually pretty good for Nightmare, imo. Just gotta know when to take the free bomb on the ground and when not to.