Type-1 energy sword/Gameplay

There are two ways a player in Halo 2, Halo 3, Halo: Reach and Halo 4 can attack, the first is to lunge with the energy sword. With this, the player lunges forward at the other player. However, this only works when the aiming reticule is lit red, indicating that the opposing player is within range. The second attack is the simple melee, this performs a quick horizontal or overhanded swipe.

Campaign

In Halo: Combat Evolved and Halo 2, use the advantage that the Elites need to be close to one to damage the player's energy shield. On higher difficulties, use the Noob Combo to eliminate the energy sword-wielding Elites. With this tactic, one can kill an Elite Honor Guard with a single Plasma Pistol easily in Halo 2, even on Heroic or Legendary.
It is extremely ill-advised to use the sword against Grunts, Jackals, Drones, or Engineers. Due to the low-threat nature of these enemies they can easily be killed with any other weapon, or even a simple melee.
The sword is extremely valuable against Elites, Brutes, and perhaps Hunters.
When facing sword-wielding Elites, a sound tactic is to jump and lunge attack them. This will kill them quickly and save one's shields or even life on higher difficulties.
In both Halo 2 and Halo 3, the energy sword is one of the best anti-Flood weapons, as it will destroy Flood Combat and Pure Forms in one strike (although if an infected Elite still has its shield, it may take two swipes, depending on the difficulty setting).
When fighting against the Flood, an attack with the energy sword will deplete the battery by a very small amount (about 3%) due to their soft tissue, making it useful for extended fights. Against Infection Forms, no power is drained.
The energy sword in Halo 2 and Halo 3 can be used to destroy the corpses of Flood Combat Forms so that they cannot be re-infected by Infection Forms. However, the cutting process does drain battery power. Although, any weapon's melee attack can be used to destroy combat form corpses if used enough times, even a depleted energy sword hilt.
In Halo 3 the energy sword is very hard to come by in the campaign outside of Floodgate, Cortana, and Halo. The player can obtain an energy sword by betraying an Elite ally on The Covenant by sticking him with a grenade (if Mythic is turned on, the Elite can survive a Plasma Grenade and his energy sword can be taken) or by draining the Arbiter's shield during combat (this happens more frequently on higher difficulties). If the player does so, it is a very good weapon against Brutes, although the battery will deplete very quickly.
In Halo: Reach the sword will knock out the shields of any Elite in one hit on Legendary, and following up with a headshot means that the sword's battery won't deplete. This is an effective strategy on Long Night of Solace, The Package and Pillar of Autumn. The sword is also incredibly effective against Hunters on all difficulties, as a hit anywhere stuns them and a single swipe to the back kills them. Opportunities for this in campaign are limited to The Package, but it is a useful tactic in Firefight.

Multiplayer

Halo 2

The sword is easily one of the best close range weapons to use in Halo 2. The best tactic is to lay into an enemy player with your secondary weapon getting closer, then when within range perform a quick swipe with the sword for the finishing move. However since there is no way of telling which player has the sword, or no sound way to stop it, it has often been labeled as a cheap tactic.
Using Active Camouflage with the sword in Halo 2 is not a wise thing to do as the sword in Halo 2 is still visible with it active.

Halo 3

In Halo 3 however, using the sword with active camouflage is a sound tactic as it will now cover the sword when picked up.
In Halo 3 the sword lunge is shorter, and the player cannot perform a quick strike. In addition, the sword's handle is now visible on the character's leg. In order for the sword to be most effective the player must lure a player into a trap where the sword will become effective; camping with the sword in a corner and waiting is the best way to use it to its fullest extent. As a special note, there is a range difference between the B button melee, and the right trigger lunge. The lunge has the maximum range, whilst the B button seems to only work at 1/2 to 3/4 the range. This is, most likely, for balancing, as the B button swings the sword at a much, much faster rate.
If the enemy controls the sword room in the Halo 3 multiplayer map The Pit, stick to the outer wall. The range is just short of the wall and if you're lucky your opponent will lunge and miss, giving you the perfect opportunity to kill him or her.
It is possible to dodge a lunge from the energy sword. To do this, one must move quickly to the right when facing the lunging opponent, towards the hand which does not hold the sword. Conversely, to prevent your target from dodging your lunge, simply jump; jumping reduces all chances of missing or being dodged by an opponent.
In Halo 3, players can parry their opponent's sword by meleeing or lunging at the same time. If the player pulls the right trigger right before the other person makes contact, a parrying electric clash is produced, but some damage will be done to shields. However, with no shields, a sword clash will not affect health, though the HUD screen will flash bright red. This is sometimes known as "fencing". If timed well enough the sword fight can last indefinitely. Because it is faster, using the melee attack instead of the lunge is required in this scenario, even for the first hit.
Jumping can make the energy sword much more fearsome, brutally effective and accurate. Jump and point your reticule down, waiting until it turns red to lunge at the opponent.

Halo Reach

In Halo: Reach, the armor abilities Evade, Sprint, and Active Camo are the most effective to use with the Energy Sword, depending on individual playing style and situation.
In Halo: Reach, there is a technique in which a player may block a sword lunge with any weapon they are carrying at the time. This is done by simply meleeing the incoming attack and stopping the attack with a well timed strike. Both players will receive possibly fatal damage from the clash, with the non-sword user taking more damage. This can be used to counter a sword user in close range if you damage them with your gun first.
Also In Halo: Reach a useful tactic is to use the Hologram armor ability and jump back just before the sword wielder lunges, your opponent will hit the hologram instead of you. This gives you time to escape or retaliate.

In Halo 2 avoiding a sword lunge is extremely difficult, if timed correctly and with a bit of luck, side stepping in the direction of the enemy players free hand may save the player's life. This is often difficult, however. In Halo 3, avoiding the sword lunge can be done by using either a Brute Shot or a Shotgun, there is still some difficulty in pulling it off, but the odds of killing an enemy player using the Sword is much greater than on Halo 2.

An extremely useful tactic against other players wielding swords, is to, rather than hitting B twice for two quick melees, strike first with a melee, and then finish them on with the lunge. This tactic is much faster than two melees.

Halo 5

As with all weapons in Halo 5, the Sword features a smart scope; in the case of this particular weapon, scoping in allows for an enhanced lunge radius. Using it is imperitive to achieve kills, as the red reticule range of the Sword has been heavily reduced when not scoped in, making spring-lunge kills far less effective. However, thrusting forward to close the gap for a lunge is an extremely useful tactic. One Warzone variant of the Sword, the Vorpal Talon, even grants an additional thruster charge to the user.
As befitting such an iconic weapon of the series, the Sword has four total variants in Warzone: the standard Sword, the faster-swinging Ravening Sliver, the thruster pack-boosting Vorpal Talon, and the Mythic Prophet's Bane, which includes a far increased lunge distance and an intrinsic Active Camo field.