Here's a few tips on various things...

I love people who use last stand. Free headshots.

yes i meant overkill as in shooting way too many bullets into you as you die... not the perk

Figured out via the hot potato trick you can do your counter-C4 and counter-claymore achievements the same way - shooting your own shit. Counter-C4 was easier - just sit around a corner with the UAV jammer perk and a silenced pistol. Put a C4 charge on the inside wall and hug that same wall (so that any enemy that comes around the corner won't see it or you until its too late) and just pull out your pistol and aim it right at the C4 pack. When enemy rounds corner, shoot. Obviously you want to change corners every so often.

You have to be a bit more precise to do the counter-claymore this way, but I only needed 2 kills to finish it off since new players always seem to put claymores near them and a frag to clear them out often kills them via their own mine.

Only have half the sniper rifle achievements to do them I'm finished them all... but godamn I hate sniping.

I got all my counter claymores and half my counter c4s by airstrikes blowing them up on people. I also have been doing counter c4 on district in the little archway next to Dom flag A. Sit in one corner and throw out the c4 in the middle. Someone always runs through there.

From a different thread

Any tips on shooting someones explosives through material and having it blow up and kill them to get the "What the ?"

get on shipment, plant c4 on the ground right next to the outside edge of a container, get your aim ready so that you are aiming through the container at the c4, wait for someone to run by, shoot...

You can also go to the rear room of the Hangar on Vacant, throw a c4 on the outside of the door, and back into the corner with your aim on it. As soon as someone walks near the door start firing through the wall onto the c4. Very easy to do if you have a bunch of people you know on your team and tell them all to go out the other way. Eventually itll flush someone there.

One thing I like to do in last stand... If there is a car nearby, I just start shooti9ng the shit out of it to blow up myself, stopping the other guy from getting the kill. Same goes for shooting my own claymores if it is facing me. This can stop the other team from getting points in a close match. Also, on team hardcore, if the game is close and you see a teammate in last stand, klll him so the other team doesn't get the points... However, this does cost you a score penalty, but does not affect your team score