Last couple of days, they've been just shooting me around the corners. I don't know what's going on. Maybe my slow connection's at it again, but I don't know how it all works.
I found a way to help combat this. Read on.
As I stated above, I started running One Man Army and Danger Close setup to unlock the challenges. I've improved my performance by a noticeble amount. Here's why and why it's related to your problem.
I have the same problem as you. Frequently, I hear somebody about to come around the corner. But, if I wait to see them to start shooting, I'm dead. The reason is the horrible lag with this game. So, I sometimes shoot before I see the person come around the corner, but it doesn't always work. Somehow, they can run straight through my bullets and kill me anyway. When I started to run my OMA and DC close, I also used SitRep which amplifies enemy foot noise. I hear them coming a mile away, if they don't have Ninja, which a lot do becaue it's the best perk in the game.
Now, with OMA and DC, I don't need to wait for them at all. I just launch a noob tube at the wall where their foot noise is coming from and it's a kill almost everytime. UAV is gold! I also check the mini map for a red dot and lauch a noob tube. I often get a kill.
Now, I know noob tubes suck big time, but it's a good way to level the playing field. If Activision didn't suck so bad, we'd have more fair games and a level playing field. But, since they don't have any interest in doing that at all, you have to do what you can.
A 'Runner' class can also work well. Just run away when you hear them coming and circle back behind them. Shoot them in the back.
From here on out, I'm only going to play as a Runner or a Noober. So, it's either Marathon+Lightweight+SitRep or One Man Army+Danger Close+SitRep.
I plan to replace SitRep with Ninja perk just as soon as I complete the final SitRep challenge.