I have trouble getting the et tu bruti achievement (Knife 5 friends) Can I do this in a private match with squad members?
The other achievement is the roadkill an enemy with a chopper. (Can it also be done in a private match with squad members)

NO You can only get the achievement in a regular match with friends on the opposing side. You will find there are opportunities to kill your [13] brothers when we are split and playing both sides. Although trying to get tags from a [13] member is not the same as tagging a random. Good luck.

its hard i know

ive done every achievemt (platinum trophy) its hard but its easy as it dosnt have to be in the same match