I'd say the pacifiers are the toughest. Pink and Blue. 

I would say impossible unless you boost.

has anyone ever got this legit?

There's no way, even a Jugernaut doesn't take that many bullets to take down!!

It is queer that Boosting is so frowned upon, but the devs put things in the game that make boosting the only viable (or most efficient)* way of achieving them

- Gold/1st place in Dom clan ops (note they have not done dom clan ops anymore - bravo)

- The emblem in discussion

- Killing people after x seconds of respawn with a tact insert

- Emblems/Titles for head shots

- Sit Rep Pro achievement in HC by destroying enemy equipment

And don't get me started about face-off games

* I am not saying you cannot achieve these things without boosting, I am saying it is much easier to get them from boosting... again... I wonder if anyone has gotten the emblem in discussion legit?