The mission "Rub Out" will be about 15 missions in, depending on how many side-missions you have done, for me it was immediately after the mission "Death Row".
This mission is not too hard. Follow Lance inside the mansion, through the back door, killing the Diaz gang members along the way. Inside, I found taking 2 flights of stairs made this mission easier (3 flights takes you to the roof). As you approach the office, Diaz will appear, and after taking a few shots, will retreat into his office, at this point, switch to the M4. This achievement will unlock during the cutscene.