Capture the Flag
From Halopedia, the Halo wiki
Capture the Flag is a multiplayer gametype where teams compete to capture a flag from an enemy base and return it to their own. Many variants of this game-type exist, with the most common variation being the number of teams and flags. More variants can be created by altering individual settings. Capture the flag has appeared in every Halo multiplayer game.
Halo: Combat Evolved
Halo 2: Anniversary
Halo 5: Guardians
Changes in Halo 4
Halo 4 featured a number of changes to Capture the Flag, which had remained relatively unchanged since first appearing in Halo: Combat Evolved. Rather than being wielded with both hands, players now carry the flag with their left hand and wield an M6H sidearm in their right, regardless of whichever weapons they are carrying. Players pick up the flag automatically by walking into it, but can no longer drop it; once a player has grabbed the flag they are stuck with it until they score it or are killed. Players can now assassinate with the flag; previously in Reach, assassinating another player while holding a flag would cause the player to drop it.
When playing with a Team of at least 4 people, a bit more strategy is required to play the game effectively.
First off, one player should be the Flag Carrier. This is the player who will capture the flag and return it to your team's base, and another would be the color guard (the person who guards the flag).
Now in CTF Classic, it is easy. Have every other teammate except for a few people defending your flag provide cover fire for the Flag Carrier. One person should (if it is available) grab a Sniper Rifle and provide cover from a distance. Every one else should have decent medium-range weapons, such as the Battle Rifle or Plasma Rifle. Also if there is a vehicle such as a Warthog, cram your team into it so that the Flag Carrier can make a speedy getaway. But to make sure the enemy doesn't come speeding after you in their own vehicle, it is advisable to destroy all of their vehicles.
In Multi-Flag CTF, the strategy is a little different. Place one defender in your base with the shotgun, and try to have the defender equipped with a rocket launcher as well. Your sniper should play from either backfield or midfield, and should have a defensive focus most of the time. A good team will be sufficiently defended using the single inside defender and the covering sniper. Remember respawning teammates can at times remain on defense to support. The key to winning any big team CTF game is communication. As long as every enemy on the board is accounted for and the inside defender is aware of enemies entering the base, any standard attack will be easily defeated. On average, roughly 50% of the attacking players will make it all the way to your base, and those that do should be easily killed by the shotgun/rocket defender. This defender must always crouch so as to avoid detection. Cover the entrances, not the flag. Other players must also listen to warnings from the sniper or defender, stating that they may soon be overwhelmed by the enemy attackers.
Offense will require careful coordination to succeed in higher levels of gameplay. Again, assuming that a good enemy will kill on average 50% of your attackers, you will need to strike with enough manpower to fight a battle inside the enemy base with half of your team. One strong way to sneak attack your opponents is to skirmish attack your enemy base knowing you will lose the fight. Keep one or two of your teammates inside the enemy base hiding and waiting to attack in unison with the next wave of attack. The enemy will underestimate your attack strength. You should use the hiding players to attack the defense seconds before your teammates arrive in the second wave. This will allow them to enter the enemy base more easily. Without more than half of the team on offense, it is unlikely that you will ever score against a strong enemy. Offensive players must also be able to survive the trip downfield to the enemy base. To do this try to (I) sneak, (II) fight (III) speed your way past the enemy. Sneaking is straight forward; travel undetected. Fighting will require strong coordination of firepower. Battle rifles are great for offensive walks downfield. Make sure that each teammate shoots together at the same enemy, killing him before moving on to the next. Think about it: three shooting enemies half dead are more dangerous than two shooting enemies and one dead enemy. The idea is to minimize the firepower your enemy is using, if you all shoot at one enemy and kill them right away, you will eliminate 1/3 of their firepower (1 out of 3 battle rifles). Speeding will require the Warthog or Mongoose, and will require you to be prepared to fight an enemy that knows you are coming.
In 1-Flag CTF all bets are off. Since you can guarantee that both forces are going to rush in full force to take the flag, you have to grab the best weapons you can manage and just go for it. It might be wise to have some units avoid the main fray and sneak around the enemy base to ambush them.
It is advised not to attempt flag running (dropping the flag while jumping) as it gives away your position for a barely noticeable speed boost. It also frequently annoys other players because the announcer will continue to say "Flag Droped, Flag Taken." It also spams the info window.
If you decide to leave one player behind to guard the flag, find out if there is any available vehicles and park them over the flag. Using this strategy, you can blow away anyone attempting to grab the flag, because they will try to grab it and instead hop into a vehicle.
An organized strategy will always win against even the most skilled opponents who do not work together.