Well, my Leixia matchup information is largely colored by my experience with SC2 Xaing, so a few things might be off...
Leixia -> First off, A,A,B without the counterhit is unsafe. You can cram in a 6A if you're on point, or a kick at the very least for a punish. 3B would only work in the Leixia is silly and tried to attack out of disadvantage. All her new guard breaks (66B,bB (?) in particular) are steppable on reaction once you know what you're looking for. You can even use 2_8 B+K to set up SCH B, which is good if the tracking doesn't screw you.
Learn to block her lows on reaction. Most aren't very dangerous, but they will interrupt you and you need to go back to blocking after that.
Her CE isn't terribly dangerous, but her BEs are. While she's got meter to burn she's got an evil frame trap/guard crush game that you're going to have to respect.
Leixia does move around which sets you up for a lot of CH knockdowns, which you need to capitalize on. Don't get greedy with the high attacks though, as her 3B is pretty much as good as ours, though it isn't as important to her. Also she does end up tech crouching a lot, so using your kicks to interrupt her, then throwing her works really well in my experience.
Her spinning attack into low guard crush (sorry, I don't recall the new command for this at the moment) you can jump. Which does suggest that 44{B}, SBH kA+B+K would dissadue her from attempting it again.
66A is a good solution to her abuse of her really good A,A.
She can't easily get under 3A or 22_88A, so those make for better spacing tools than 6A or agA in this matchup. Also throwing 3K and 66K out on occasion does a good job of checking her.
Astaroth -> I wrote some stuff over here:
http://8wayrun.com/posts/419724/
Nightmare -> I'll preface this with the fact that I've never played a tournament caliber Nightmare, so this is going to be a bit superficial...
Being able to block his basic strings on reaction will help a lot here. He's just as telegraphed as we are, and knowing his lows are coming and punishing with 44B will pay off big.
Grim Stride isn't safe. You can hit him out of the transition pretty easily unless he uses a guard crush to cover it. His GS A will GI verticals, so you'll want to use kicks or 6A and the like to stop him. Alternatively you can block whatever he throws out of the stance and flapjack fairly reliably.
His ring out game is incredible, but since he uses the same tools we do in his game, he's just as vulnerable to a flapjack out of the ring as we are. Even better, since he'll be intimately familiar with the flapjack, he'll often open himself up to 1B at the ring edge. Make him suffer.
Don't try to hit out of disadvantage after his agA, treat it just like Siegs, only prettier. If he uses it a lot, go under it with 3{B}.
Hit him out of his stances. As Sieg players we should be pretty good at recognizing them. His fastest stuff out his side stance is high, so 3B is a good choice. Don't step it though, as he has a mid horizontal that rings out on CH.
Give him some respect when he's got meter charged, as waltzing into his GI CE will hurt, but keep in mind that it's only 40%, so if you've got a huge life advantage, you can make him burn it. And if you bait it out of him without hitting the GI, just step the move and punish.
6A is a great interrupt this fight, but it is punishable so you have to be smart about when you play it.
22_88A is your spacing move of choice here. You'll find yourself in the appropriate range for it alot if you're getting knocked around, and all the move's specs are favorable.
You can punish a lot of his lows with WR {B}. That's often a good time to spend meter for the extra damage.