Flatten out your back swing. I have recently been crushing all of my longer irons just from this simple 'fix'. Slow your swing tempo down and take the club back lower and a bit to the inside. Make a conscious effort to do so and I promise you will see SOME amount of improvement. IMHO, most people can't hit a long iron because they are coming in too steep to the ball at impact. If you will try to swing 'around' your body more instead of 'up and down' it will help. Don't worry about 'hitting down' on to the golf ball, you are going to do that inevitably. Hope that made sense and helps. Good Luck!!