Overkill (achievement)
From Halopedia, the Halo wiki
Overkill is a Halo 3 multiplayer achievement. It is awarded when the player gets an Overkill in a ranked free for all playlist. This Achievement gives 5 Gamerpoints to the player. Overkill also unlocks the shoulder piece for the Elite Commando Armor and shoulders for the Elite Ascetic armor.
It is represented by a bronze medal with four green stars.
Difficulty
This Achievement has been antagonized for its difficulty to earn. Currently, there is only one playlist that falls within the conditions. Since the playlist, Lone Wolves, originally only supported 5 player per match, for Overkill to be unlocked a player had to kill every other player in the game within four seconds of each kill. This was proven very difficult, and many players opt to get this achievement simply in an achievement match. This was rectified and made slightly easier in a recent update, where the amount of people allowed per game was upped to 6.
It was briefly possible to earn this achievement relatively easily during the Halloween zombie special. If you started as the alpha zombie on a "creeping death" match, it was possible to have four or more opponents clustered together and to easily sneak up and eliminate them all with a single gravity hammer stroke.
An easy way to do this will be to man the turrets on the pit in a crazy king match, people in the hill will usually be to preoccupied to worry about you and you can mow them down with ease.
Another easy way is to take a sniper on king of the hill games, stay far away from the hill, and pick off the people on the hill
You can also do this with swords on a small map, like Epitaph. Since Epitaph has a "Main Battle area" (inside on the 1st floor) where all the carnage is at, simply go there and try to sneak up on people. Sword clashing is the negative side to this, though. Sword clash battles can take more than 4 seconds to finish, which violates a requirement for this achievement.
You can also try tossing in grenades while your enemies sword clash each other. If the grenade is thrown right and their shields are down (from sword clashing each other), It should land you at least a double kill.
A popular way to gain the achievements is to grab a shotgun or gravity hammer during a Crazy King match. The rushing players will probably be caught unawares, which will prove a bit easier, although if a another player is near the player you just killed, its going to be a 100% chance the player will be backpedaling and firing away.
A tricky, but possible way to get the achievement is to get both a Shotgun and a Bubble Shield. First, place the Bubble Shield. This will act as a magnet, attracting players like a sign that says, "There's a kill." As soon as they come, give them a nice shot in the face with your Shotgun. If enough players come within a few seconds, you could at least get a Triple Kill. Beware of swords.
