On Slayer, go on the map Makin.

Go to the tower by the water, when you get to the top (but be careful, the wood is not bullet proof) and so when you head to the top, jump on the sand bag on the front left corner.

You'll see a very thin wall, it'll take some time to jump on the wall.

But when you nail the jump, head right until you see a little platform.

That's the hidden killing spot.

It works really well with M1 grenades and shotguns.