Sole Survivor Achievement
Sole survivor on your multiplayer team.
In any team mode where you have someone else on your side, you must be alive at the time they are dying. If there are multiple people on your team, they all must have died and you be the lone survivor. This is easy to boost.
Online private match boostable (3 people required)
Start a private match up with 3 people total. You and one person on a team, and the other by themselves. The player on the opposite team kills your team mate. That's all you have to do.