This is a guide on how to get the 'Killer's Spree', 'Killer's Rampage' and 'Sniper' achievements in rainbow 6 vegas. May as well go for them together to save time.
Just need you and friend to boost
Change your weapon to the sniper rifle and set up a ranked match and get your friend to search.
These are the settings I'd suggest to host.
Game Mode - Sharp Shooter
Map - Calypso Casino
Round duration - 10 mins
Players - 12 (Doesn't seem to find people quick with two)
Rounds Per match - 1
Respawns - Yes
Now just kill your friend 10 times in a row with the sniper rifle. We just met at the stairs as it was some where we both know.