some of my thoughts of this match up
Against Astaroth, i prefer to limit the use of WSB, instead use WC3B w is still punishable but at least it does decent guard damage, you got to keep in mind things that can lead with the BE bullrush, consider this thing is a big threat against Nightmare getting that fucking titan bomb (28 B+G) after getting relaunch, thats horrible damage.
i noticed after BE bullrush revenge 3K Nightmare is able to block, 66K 6 is pretty nice cause it can be Revenged by BE bullrush but if you see the revenge effect is activate you can auto GI with the GS A, it is confirmable, and considering 66K is TC, i think fast TC moves like this and 1K are annoying to Asta cause of his lack of good mid interrupters, maybe his 6K w his unsafe, and his 1K and low throws, keep in mind although 3B is punishable by bullrush, the revenge doesnt affect it, GS B neither, flap jacks and over toss are great he cant punish this decently.
Try to predict his BE bullrush, if you block it you can easily step it to Nightmare's right side and punish with GSB (is a bit hard to punish with this one but not so hard) for a lot of damage, or if you prefer more easily with 4A, 4K BE:B for 102 dmg or a BT 4K BE for 98 dmg.
hope this help you in this match up but i still thing he is kinda annoying to Nightmare