EDIT: Since the 11/16/17 the achievement is finally obtainable with the new update if you're stucked at 99% as written below ! You just have to launch your save and the cheevo will pop just after the loading screen :D
Hi ! If after beating all missions,found all colectibles and bought everything in all shops you stiil need 1 or a few collectibles (especially skins) i'm sorry but this achievement is currently unobtainable.
In my case,i've 13/14 weapon skins and the Pink one didn't counted for me.
Happy hunting guys !