Mastery Assignment too hard/impossible (the last one)

No argument from me, the assignments are one of the things I find frustrating about BFV. I strongly believe the cumulative 'hidden' option, which was actually a bug, shouldn't have been removed. At least if that was still in the game people could grind it out. Nowadays I don't even attempt them, they were starting to spoil my enjoyment.

That said I'd take the headshot kill assignments on the bolt-action rifles over the matching assignments for SLRs and SARs - at least then you don't need to worry about having to score 2 - 3 hits to a head (might be less of an issue on PC of course).

@donthurtm3 I do agree, that some of the assignments are to hard and the community has been complaining about that since the release of the game 3 years ago. As they have not changed them since then and the development of the game has been ceased, I don´t think we will ever see a change.
However, the assignments you mentioned aren´t even the really hard once as @ElliotLH said already. The Bolt Action Assignments and especially the Assault Rifle Assignments are actually quiet doable, you just have to choose the right situation to do them. For the once you metioned, I´d go for one of the linear gamemodes like Breakthrough, Frontlines or Rush. In these you mostly concentrate on a single objective (maybe 2) and you have a lot of enemies sitting in the objective area. Especially, being on the attacking side in Breakthrough, makes the Assault Rifle one quiet easy, as you are constantly attacking objectives.