Noob Combo
From Halopedia, the Halo wiki
The so-called "Newb Combo" is the combination of the Plasma Pistol and Battle Rifle or the Carbine. Often one would be walking and hear the splash of the plasma pistol being fired only to turn around and be hit in the face with the giant green blob and then sprayed in the head with the Battle Rifle/Carbine in an instant. This combo was especially rampant on Beaver Creek (Level) and Colossus (Level). It was incredibly effective and often turned the tide of games. This tactic was considered so imbalanced that Bungie attempted to remove it from the game by implementing the new Battle Rifle game type which removes all Covenant weapons from the map. This was largely ineffective. They also made a team actionsack gametype for it called "combo training"
Whether or not this weapon combination is actually a sign of being a "newb" is up for debate. While it is indeed a powerful and arguably imbalanced combination, it does require some skill to use, and using weapon combinations effectively is not a trait of a new player.
However, even a player who would normally lose to you can win using this combo. Another effective combo is a Magnum and a Plasma Pistol, one in each hand. It is usually your doom or your savior.
Blocking It
With a great deal of skill, patience and practice it IS possible to block the noob combo!
Here's what you do:
You MUST be dual wielding, and facing your opponent. Wait until the plasma discharge is about to hit you, then immediately press B, or melee. If done correctly, and properly timed, the plasma discharge will hit the weapon you just threw instead of you! It seems best to drop a plasma rifle.
A second method that can block the noob combo is to hit the discharged shot with a grenade. Plasma and frag grenades both work, and Im not sure which is best to use. Its argued that a plasma is better because it is a glowing ball of energy that the disharged bolt will home in on the grenade instead. My personal account is that frags are better because they fly throught the air faster.
Both the duel wielding and grenade method work and its about preference on which to use
Survival can't be guaranteed, but it gives you an edge if you can master this slick move!
Other Names
Anti-Newb Combo, Newb Package, Dirty Combo, Dirty Package, Bitch Combo, Bull-dogging.