To get the Snake camo you have to defeat the Boss by depleting her stamina. An easier way to do this is by using the Snow facepaint and camo and ducking, she will star looking for you and that will be your chance to use the Mk22 or the End's sniper. If she finds you and shoots, run and roll then stay down, she will run away and lose you. If she gets you in CQC, press Circle repeatidly to prevent being damaged, instead of using CQC against her try punching her, when she falls shoot a tranquilizer to her. Once her stamina is depleted, she'll stay down and the camo will come out of her, look nearby for the camo the box if bigger than others.