Banzai Charger

Banzai chargers are Japanese soldiers that will rush at the player with bayonets attached to Arisakas during combat or when attempting an ambush, (as in the level Hard Landing) yelling "Tennōheika banzai!"("Long live the emperor!") or just simply "BANZAI!!". Japanese soldiers were indoctrinated to believe that surrendering was dishonorable and cowardly, and to choose death over surrender. Thus, banzai chargers are often a final stand in the face of defeat so that they could die with honor.

History
Banzai charging was an extremely effective maneuver against most forces, however, eventually the charge became ineffective to the Chinese' superior melee training and the well-trained American forces armed with semi-automatic and automatic weapons. This tactic was first used against American forces during the Battle for Attu islands by Colonel Yasugo Yamazaki of the Japanese forces garrisoned there. Towards the end of the battle, he gathered all the remaining Japanese forces on the island and charged at the U.S. forces with a katana in hand. Although only 28 Japanese soldiers out of the 1,000 were taken prisoner (none of them were officers), the attacked penetrated American defenses and shocked the rear-echlon units. The largest banzai attack was on the island of Saipan the night after the initial landing and was so big and unexpected that the Japanese got all the way to the beach and almost stopped the invasion right there but were beaten back by stiff resistance. However, most of the Japanese commanders chose not to use massive Banzai Charges during the war except as a final attack to avoid capture and dishonor. During the war, American soldiers usually made short work of Banzai Charges with automatic weapons and suppressing fire, although this was less successful during the night.

Part of the Information taken from Wikipedia.

Defending Against Banzai Chargers
Banzai Chargers will often jump out of tall grass, spider holes, jungles (as seen in the level Hard Landing), out of Japanese defenses, or they pretend to be dead. They will often attack when least expected, such as when a player is in the middle of reloading. If a charger reaches you, he will push you down and try to impale you with a bayonet. To stop them, you will have to press a button (varies for what console you are playing on) before he stabs you so that you can push his rifle aside and stab him in the neck. For players who cannot do this at an accurate time, they should repeatedly press the action button as soon as you get pushed down. Players can also shoot the chargers, or use their knife if they time the attack right (this often results in the player taking some damage as well).

On the Xbox 360 and PS3 version, players that successfully counter a banzai charger for the first time will unlock the achievement "Close Shave" worth 10 Gamerpoints. You can also charge at them too if you have an Arisaka or M1 Garand (in the level Little Resistance) equipped with a bayonet if you want to make it look like a Civil War bayonet clash

Getting the Achievement
The key to getting this achievement/trophy is purely timing. As soon as the attacker fully draws back his bayonet, (About a split second after being knocked over) click the Right Stick to stab the attacker, and survive.

Glitches

 * Sometimes the player is knocked over by a banzai charger but instead of attacking, he reloads his Arisaka, and the player simply gets back up.
 * It is possible, when knocked over, two Banzai Chargers will be seen: one will be stuck reloading and the other attacking.
 * If you shoot at the right time the Banzai Charger will disappear and you will be lying on the ground staring at the sky, unable to move.
 * You will always see your hand being stabbed by a Banzai Charger while pushing his rifle aside, however, this doesn't damage you or affect gameplay in any way.

Trivia

 * In Co-op, you will not be knocked down.
 * Sometimes the AI will get into fights and will bash the Banzai Charger with their weapon or flip the charger over and shoot them point-blank.
 * Sometimes, soldiers without bayonets will charge at you shouting "BANZAI" but when the reach you, they will hit you with the gun, rather than knock you down.
 * You can use bayonet rifle to attack your allies,too. They will scream like a Japanese and will have the same death scene.
 * In Hard Landing, when the one of the soldiers says "Aw Shit, Basterds just took out another one!", a Banzai Charger will say a long sentence and then charge at you.