That's the worst, you think you're sneaking up on someone then *schnik* and it's all over!
I think the main one you need a friend for is the x number of times your teammates spawn on you without dying. Some of the badges and awards look pretty tough too.
The only one I want is where you invite a friend into your game whilst they are playing BF and they join you for at least one round. Sounds easy enough as we were all playing the other night anyway!
Well I'm up for a game tomorrow night. Working till 9 so not seeing the GF which means plenty of free Battlefield time after dinner.
I'll probably be on around 10 if anyone wants to join me.