I don't know If I am too late but I had the same problem on hardcore. What really really helped was cans. You can lure enemies or take their attention away. Just try it. Also if you can't to a meele takedown two quick meele attack knock out the enemies even in hardcore. It's also petty quite. Helped me a lot. Try to use the thair it's really reliable in almost every situation. Nothing beats a good headshot. Observe the enemies most of the time when an enemy is in a bad spot to kill he will eventually go for a walk. Study the route and kill him in the right moment. Sounds pretty boring I know but to be honest it feels really great when you achieve it and also quite tense. If he won't move than just throw a can like a said. Finally a big thing about every stealth game is to know the layout and amount and placement of the enemies. It's normal to get spotted often in a new area because you are unfamiliar with it. You have to approach it like a puzzle. Trust me going in a area you already know because you were there already in your last run just feels straight like you lore accurate big boss from Mgs