Monday Night Combat

The best way to get this achievement is by selecting Private Match from the Xbox Live menu. Have 2 controllers on the same Xbox. Put the one getting the achievement as Gunner (obviously) and the other controller as Assassin (she has the least health, easiest to kill in one hit). As the Gunner, go to the enemy base, just in case you don't do it first time. Stand by the assassins spawn point, walk her out. Jump over her and press X to do the Ground Slam attack. If you did the attack directly over the opponent, she should be killed in one hit. This achievement can also be boosted with a friend.

If you can tag along with a Sniper ,while he uses his Freeze trap the enemy can not move while trapped so fly over his head SMASH!.

you can be in a private match for this achievement.

as the gunner, get next to and weaken the person youre trying to ground slam so that you can ensure a one hit death. jump up in the air and stay close to the person youre about to ground slam. the gunners knees should be near the opponents shoulders. then simply hit the X button. you should see the opponent get flattened into a pancake, and achievement then unlocked. takes a little practice, but can be easily done

you can do this by jumping off a ledge, but i found that its harder to do, since the slam needs to be accurate.

This is a little bit difficult achievement. It requires you to do the Gunner's Ground Slam ability (X) and kill an opponent under you.

First I recommend upgrading the ability to level 3. Not required but helps a lot. Next position yourself on a walkway that has a tunnel underneath. You have to jump (should be A), hold the button to hover and hit X over the opponent. If you hit them directly you got the achievement.

You'll know if you hit them if they die right away and not fly back from the shockwave.

Just like joe said, this can also be done in a private match with a second controller.

For this you have to use the Gunner in Crossfire and his Aerial Ground Slam to flatten an enemy player. This also unlocks a pro-tag

The Gunners Ground Slam ability is a very effective one. If you are trying to get this achievement I would suggest you try and have the Skill Recover endorsement for Gold in case you miss. Getting this will take precision timing or absolute luck (I got lucky). If you can get to a high point on the level try and stay there and wait for someone to pass under you. Those of a Guitar Hero/Rock Band disposition might do well, simply predict the opponents movements, jump and hit X (default controls) and they should have died easily.

another addition to the above posts is once you unlock your first custom class in the locker room, choose the gunner class and the endorsements Sprintz Gold, GobiNumb Silver and Smedley's as bronze. This should give you a lot more speed and armor so that you can get into a position sooner as well as give you some ability to chase other players should they move into a different location.

Also if the enemy team has an assassin running around, if they grapple you from front on, quickly hit A and then X for the gunner to do the slam as he is getting up off the ground, more then likely the assassin will stay close or move closer due to wanting to finish you off with their dagger or sword and the quick button mash will end with you squashing them into the ground.