I've only got 4 regular challeges left to do....(Aside from the Headshot Challenges)
They are:
Counter Claymore
Return to Sender
Hot Potato (Every fckin' cooks their grenades now though)
Invincible

(feckin difficult too)
I managed the How the? last night and I've trick to doing it.
Throw some C4 on the first landing of a flight of stairs,
wait at the second flight of stairs and aim your cursor at the C4 through one
of the upstands for the Banister rail.
Then just wait for someone to come along. Gotta be quick though or
the gey auto-aim'll start targeting the enemy instead !!
Anyone else got 'em all done or nearly all done ??