Anytime a decent portion of the ball is sitting down in the grass, you have to change your angle of attack on the downswing to be sure you hit a solid shot. When I'm playing a full iron shot out of the rough, I make a swing that is considerably steeper than normal. I do this mostly with my wrist hinge on the backswing.
I feel as if I'm swinging the club straight up off the ball and then almost chopping down on it. This steep angle into impact is the key to making contact with the ball before the clubhead gets caught up in the heavy grass.
I usually play the ball a little farther back in my stance, which also helps me make sure I hit the ball before the grass. And then, as I start back, I set my wrists immediately.
I recommend playing one club longer than normal and allowing for the ball to fade a bit because it's harder to square the clubface at impact from this ball position.
Ryan Moore won the 2009 Wyndham Championship and the 2004 U.S. Amateur.