I dont have any of these acheevos, if anyone doesnt have them, or if anyone wants to help me out send me a friend request, thanks, i gotta mic
I don't recommend you to do those with a partner. You see, the less people there are, the more points one player gets. They are quite easy, I think you can get them no problem even on normal.
I can help you if you want but first consider the following:
It's quicker by yourself just do this:
Skulls: Tough Luck, Catch and Fog
The time multiplier is useful if you just want to get 5k (or 17k in the case of The Ark and The Covenant) and run for it.
This is more than enough to get you 15k but if you want someone to tag along I'm in.