The videos definitely help... And some of this has been said, but here is my take:
1. I'm sure you know to grab the piston from the "get the sniper in the windows opposite" section.
2. And when you get to the Amsel part, take out his bodyguard/sniper ASAP (I thought there was only one, but I read two in the other guides here).
You have to shoot Amsel twice (with the pistol).
I found the two best/clearest opportunities for this is when he:
1. Make his first dash from behind the tank to the truck, and
2. When he runs from his car (snipe the driver).
Between these two times a truck with a mounted machine gun will arrive. Take this gunner out ASAP, else it makes your task just that much harder.
I was getting very good at making one shot or the other, but it took forever to get both.
When I finally did, I actually got both shots when he ran from the tank...
So basically, just keep at it.
I was getting to the point to figure out how to use Amsel's shadow, which might help, but luckily I got lucky.