When changing to the D chord, place 3 down first, before removing the 1st. This allows you to maintain the heavy arm. Similarly, hold the 3rd in place on the D chord until the index is down for the A chord. 

Bar 2: While still holding 1 on the A chord, place 3 lightly on the first note of the run, then release 1 from the A chord. 

Practice this run using a metronome, and all the power of The Principles you can muster! Watch your pick, and master this run PERFECTLY slowly first, using the metronome with the Basic Practice Approach to reach your first goal, perfection at 60 bpm, one note per click. 

Then, work it to 60, two notes per click, then, 60 in 16ths. Then work to performance tempo.

Note: the bent note in the run is bent with the 2nd finger, but the 1st helps out, placed behind it on the string. 

Bar 4: This run requires some stretching in the left hand, be careful of sympathetic tension throughout the body during practice. Watch the pick as you practice and be SCRUPULOUS about the alternate picking. Make sure your pick strokes, even when done slowly, are done with great force, and relax in between the notes. (read Tech Tip #7, "May The Force Be With You"). Again, your approach must be "no tempo, then slow tempo, then work up with the metronome. 

Good luck, and make Angus proud!