Truth's post in Weapon expert 7 was marked as the answer
I noticed that some of them don't really track correctly, mostly the ones that require you to do things consecutively.
It may help to make sure you don't have a rifle with you during the attempt. I think I got this one at some farm. Just take your time and try not to pull out any other weapons.
Truth's post in Weapon expert 2 was marked as the answer
What's the challenge? Is that the knives?
Edit: It is!
For that one, I found a group of people (probably some gang's camp) and snuck up on them. Line up the headshot, then throw. As soon as it's in the air, line up the next throw (auto aim is a blessing here) and go into dead eye. Throw and repeat for #3.
I expected this one to be difficult, but I actually got it on my first attempt.