Everything about difficult MILs lets us know how not to be behave when our DSs get married.
I always feel toxic and depressed after my MIL visits. She bitches about almost every family member we ask after. It's not nice. And it's not nice for DH to see that his mum is in fact a bitch who prefers to say nasty things about people. We tend to keep quiet and not say much but she still bangs on about people and the wrongs they've done her over the years even children who have apparently wronged her other grandchildren.
Like yours, Jaynee, she would much rather I was out of the picture and she could have DH and my DCs to herself. That's tough really on both you and her. You because that's a pretty sh*tty attitude to have and on her, because it's not going to happen.
I wouldn't bother standing up to her per se. You can quietly be assertive without being confrontational. You can thank her for her advice but you'd rather do things this way. Constantly tell her how well DD is looking, how well she's doing at school etc. If she says anything negative, just ask her if she's saying DD is less than perfect. If she's saying you are less than a perfect mother, then say you'd rather she said nothing if she's nothing nice to say. And especially in front of your DD because you wouldn't dream of slagging off family in front of your child.
Bear in mind she's probably difficult because she feels threatened by you and what a great job you're doing with your child.
It's tricky I know. And all this BS about you married your husband therefore you married his parents - cobblers. But you just make sure you maintain your dignity and your parenting preferences - you never married to give those up. The rest of it doesn't matter.