I just wanted To add, that if you dont care about losing everything you had achieved in the came (linke Lvl. 50 and your guns etc.) and only want this achievment, than you can delete your gamingprogress in the xBoxmenu/settings and than play from the beginning as a Lvl.1 and in the Mission 1-1 and 1-2.
I think, its the fastes way to geht this achievement if you where Art a high lvl or tried the guieds above.
That way, after trying the other ways above, which didnt work out for me, i only needed 1 try To play the first 2 missions after it worked.
Hope, it helps. ^^
And please leave feedback, i would like to know if it works for others too. Thx