The magnum's also really nice against these, one headshot and they go down. Any normal machine gun type takes a whole lot of headshots, but if you're at a good distance sometimes they can't hit you, so you can just wait way back and peg them off. Also, they're not very good at hitting you if you're moving sideways and changing direction all the time between their shots, so you have a chance up close too, but it can get very difficult.
Yeah, but by the time I get that close, screen starts to get blurry...
I had a really hard time getting past the courtyard of the last mission, the guys with shields chased me into the building (trying to hid and snipe), and even grenades don't help that much. The only thing that worked well is RPG, and I was trying to save them for later. So finally had to use RPG.
For those who don't chase, they are pretty easy to handle.