The Brink:
I was getting shot at and just lobbed a greanade in the direction of the bullets and took out 3, not really something you can practice though, just lucky.

ClayMORE.
Put claymores in high traffic areas like the stairs near the house & van in district or in the hallway of the house that has the mounted gun upstairs in overgrown.
My only tip with claymores is not to put them too close to the corners as peole get used to this, put them a bit further back so they will have already come around the corner before they set the claymore off, then it is too late.

how did you get the 'What the ??' challenge? I notice it isnt on your list, I have no idea how to get that one.