My best advice to you is to keep her away and exploit the gaps she has in her 50/50s and strings. She gets two 50/50s everytime she touches you, the first one is unreactable, but the second one is pretty weak tbh. You can armor through her grab and cartwheel, armoring through them is a guess too though because the overhead cartwheel stuffs slow armor (like Erron's MB sand in Outlaw) and you can't armor through her low if she does it immediately. If she delays the low and your armor option is a high, it'll whiff, so be careful. She has a gap in her b1*4 string that you can backdash or armor through depending on the space. And most importantly, PUNISH HER 50/50 if you guess right, they're all unsafe.
There's no character I feel constantly kicks my ass, but my vote is for Johnny Cage just because I hate that character in general. Everything about him, from his fighting style, to his god-awful personality, to even almost every fatality he's ever had. I admit though, I love his first Fatality in MKX because it's funny.
Not because I always lose to him. Its just because I know my opponent is going to suck, but I'm going to have to play well to win because KJ is braindead.
Mileena by far. Most people I run into online only uppercut after roll so it's not that bad damage wise but online Mileenas are just the worst. Yolo rolls, random x-rays, Ball into Ex Ball because of the netcode. I will gladly fight 200 predator players in a row in order to never fight a Mileena player online again.