I now done all the challenges, 100% on all maps now. Just need to do multiplayer, rope pull 5 enemeys, loot enmines, amongst others, so i'll have to do a replay I think.
75 kills with a rifle is quite a few. Perhaps you're really close. I don't know why the rope pulls aren't working for you. If you pulling them off a platform and they are dying you should only need 5.
I saw you over at the Edios Tomb Raider board and that is a very active one. Hopefully someone can help you there as well.