From Halopedia, the Halo wiki
Noob Combo is a term coined by multiplayer gamers shortly after the release of Halo 2. However, the original idea came from Halo: Combat Evolved, albeit a modified version. It is typically used to refer to the combination of a Plasma Pistol overcharge and a semiautomatic or burst-fire weapon, such as a Magnum or a Battle Rifle, though it can also refer to other weapon pairings, but not as commonly. The idea is that a noob combo can be used in multiplayer games to gain quick, easy (and arguably "noobish") kills.
The basic technique is to sneak up on a player and hit them with a Plasma Pistol overcharge. The overcharged plasma bolt will drain the victim's shields, allowing the attacker to finish them off with a headshot. The most common noob combo is the combination of a Plasma Pistol and Battle Rifle. Another popular combination is of the Plasma Pistol and Magnum -- dual-wielded, of course, to remove the need to switch weapons—as the Magnum is sufficiently accurate at medium range.
Halo 3's Plasma Pistol was altered; the overcharge shot's homing ability was weakened, and the projectile's speed was reduced. A new feature was also implemented: "holding" an overcharged shot will continuously drain a Plasma Pistol's battery. This effectively caused a drastic decrease in the use of the noob combo.
In Halo 3: ODST and all subsequent games, the lack of dual wield makes this technique slightly harder than before. However, in the Campaign, enemies are usually stunned when their shields are drained, giving the player time to switch to a finishing weapon.
Whether or not these weapon combinations are actually "noobish" is up for debate. Detractors of these combinations have argued that they are overpowered, unbalanced, and do not require much skill. Proponents of the infamous "noob combo" argue that a truly skilled player can survive a noob combo and kill anyone who dares to try one, even though this is highly unlikely due to the speed of the combo and the unfairness of the defender having been instanly drained of their shields. Some people believe that 'noob Combo' should refer to the action, not the weapons.
In Halo 2, a player can block a noob combo through various means. One method involves a defender who dual-wields weapons and melees when the attacker's overcharged Plasma Pistol shot is about to hit. The melee forces the defender to drop a weapon; if timed right, the attacker's plasma bolt will hit the defender's dropped weapon as it falls. Plasma Rifles work well, due to their bulky heads.
Alternatively, a well-timed grenade can block an overcharged Plasma Pistol shot. Plasma and Frag Grenades both work, though it is not known which is more effective. Some argue that an overcharged plasma bolt homes in on Plasma Grenades better, while others contend that Frag Grenades are thrown faster. In Halo 3, this counter-strategy (when used with a Frag Grenade) has the added bonus of injuring the opponent—if the defender's grenade is thrown just as the plasma bolt is fired, the bolt will detonate the grenade in front of the attacker.
When the aim is to stop a combo in progress (after having been hit by the overcharged plasma shot), a defender can lob a Frag Grenade at the attacker and then crouch. The attacker's attempt to line up a headshot will be slowed down by the defender's crouching, and the grenade will hopefully distract the attacker long enough for the defender's shields to recharge somewhat. Plasma Grenades should not be used, as they take longer to detonate.
Despite the negative stigma surrounding the Noob Combo, it remains a very common tactic in multiplayer. It is also extremely useful when playing the games' Campaigns and Firefight on higher difficulties, especially against powerful enemies such as Zealots and Spec Ops Elites. It is of limited use against either varieties of Brute Chieftains, but against the rest of the Brute ranks found in Halo 3 and Halo 3: ODST it is devastating, removing their armor making the follow up headshot, usually lethal. Halo 3: ODST encourages the latter usage, granting players an achievement for earning ten kills with the combo. In Halo: Reach, the noob combo is less effective against high-ranking enemies. Sangheili Zealots and Brute Chieftains gain shields that can withstand multiple overcharges, and are prone to dodge the projectile despite its homing effect.