Today the only letter i needed was I and i got it in the 35kill at mexico S&D well, the only thing that has worked out for me is this:
First you need to start completing missions and since the 1st mission is always the same for 1 month you do it 2 times so u get 2 letters, then you copy the letters u get on a paper, always put them for the type of game, ghost mode, FFA, TD, S&D and like that.
Then do other missions and when you are down to 1 letter look at your paper with all the letters copied and look which one gave u that letter the most ok now a example so u get it lol
Example: Ghost Mode: I, S, S ,S ,I
FFA: O, R , R, R2
TD: F, C ,S2, I
ok lets say that u have got that in a paper and the letter u are missing is S, then you should do Ghost mode mission and you will likely get it.
THIS IS NOT 100% PROOVED but what i always do is try to do a different game mode every day so i try getting different letters
Today i got my last letter and won D.e Scope
and like 20k gp I'm awesome weeeee