For the VR missions only missions 18 and 19 are the only difficult ones to complete on my end. For mission 7, I used Sundowners weapon and just charged attack with it.
But if you want a guid on just completing them and not getting the highest rank on them use this.
http://www.xbox360achievements.org/f...d.php?t=425067
Seeing this player getting the highest on each VR mission rank kind of makes me want to go for it too.