From Halopedia, the Halo wiki
The Warden Eternal is a formidable foe who is equipped with his unique dual-bladed lightblade that he uses to strike down his enemies. He also has a built-in directed energy weapon that emits a powerful hardlight beam from his head for ranged engagements, as well as a gravity bomb, a slow moving but powerful homing attack that can deal serious damage to a moderately armored vehicle, and a powerful shockwave-like attack that can blast nearby objects away from him by several tens of feet. None of his attacks can instantly kill a campaign player character and make them unrevivable (assuming the characters are controlled by human players), but they are otherwise still extremely powerful. However, his stomp and head beam attacks can prove fatal to vehicles as they can temporarily disable them with an electromagnetic pulse effect.
When damaged enough, he is stunned as one of his vulnerable cores is exposed. There is a large core on his back that acts as his main weak point. Once he is destroyed, a slipspace portal forms around his torso and pulls in the remains of his body before vanishing.
The Warden Eternal is featured in the following missions:
When fighting Wardens on Heroic or Legendary, make sure your teammates (whether NPCs or not) are equipped with heavy weapons (e.g. Incineration Cannon, Railgun) and keep ordering them to stay close to you (only works if playing as John-117 or Jameson Locke). Kill any nearby Prometheans and run to cover as soon as the Warden activates his gravity bomb. On Normal or Easy, simply blast him to pieces with the Incineration Cannon or any other heavy weapon.
If engaging him from far away (e.g. in Reunion), have a LightRifle or DMR ready alongside either a Railgun or Binary Rifle. The Warden can still be damaged even if shot anywhere besides his weak point, but with any long-range weapon, make sure to be scoped in at all times and avoid enemy fire from Prometheans nearby and/or below.
Warzone / Warzone Firefight
The Warden Eternal appears as a boss in Warzone and Warzone Firefight, in three forms: the standard form, marked as a Legendary boss; a more powerful Legendary variant, with orange coloring with teal highlights; and an extremely durable Mythic variant, colored pale red with blood red highlights.
In either of his forms, the Warzone versions of the Warden are much more potent and durable, with the Mythic form having a gravity bomb that exhibits slightly stronger tracking behavior and the ability to shoot hardlight beams more quickly.
Regarding vehicles, the Phaeton Helios is the most effective choice. Banshees are also effective, but often have to make multiple runs against the Warden, who can temporarily disable them with his shockwave. In any case, Banshees and Phaetons can perform evasive maneuvers to avoid being temporarily disabled. Scorpions and Wraiths are not bad choices, as they are actually capable of doing sizable damage to the Warden, but are rather slow, leaving them more vulnerable. Using a Damage Boost also helps, as Damage Boosts now apply to vehicles, unlike in Halo 4.
If fighting him on foot, it is highly advisable to pick up an Overshield or a Damage Boost and a strong power weapon (e.g. Talon of the Lost, River of Light, Twin Jewels of Maethrillian). However, a lot of dodging is required to avoid dying to the Warden due to his powerful attacks.