This is a description of the Warden Eternal from the Game Guide:
A towering Forerunner warrior, the Warden Eternal is an extremely dangerous adversary. It is heavily armored but features a glowing weak spot on its back. When battled at range, the Warden casts out large, slow-moving orbs that home in on their target. Should an orb find its mark, the target is quickly obliterated; the blast is so severe that it can even penetrate thin forms of cover. Battling the Warden up close is equally risky, for the construct wields a massive blade to great effect. Either keep distance from the Warden and remain near thick, effective cover as you pick shots at it from afar, or thrust to avoid its melee attacks as you look to counter with close-range blasts to the Warden's vulnerable back.
(pg. 119)
Though not mentioned in the description, the Warden Eternal has a third weapon: an eye beam.
The Warden Eternal is encounted on Missions 5 (1x), 7 (1x), 12 (1x), 13 (2x), and 14 (3x).
The guide has to say about fighting the Warden Eternal on Legendary,
If you're playing with friends, spread out and flank the Warden from all sides for the best odds of striking its vulnerable back.
(pg. 119)
In this game, the commands for the squad are:
1. Move to a location. (Full Squad)
2. Attack a target. (Full Squad)
3. Take a weapon. (Individual squad member)
4. Revive. (Full Squad)
Note: the order can be cancelled by the squad if they are "busy" or if you are not on their "path" - be in a location accessible to the player, not necessary so for the squaddies.
Another note: the AI of these squaddies is average of video games. They are not exceptional in anyway.
The sniper's perch remains advantageous during Legendary play, but you'll run out of ammo before long. Empty the Binary Rifle on the Warden (aim for the head), then finish him off with a Lightrifle.
(Pg. 142)
In the first two encounters, the Warden can be fought from height. This helps as the Warden is not capable of scaling heights. However, its orb attack and eye beam can still cause damage, so I would fire at him, duck back into cover when I saw the orb, and come out of cover to fight it again. The orb is a slow moving attack and can be avoided. The eye beam cannot be avoided. In the later missions, you are playing the Warden Eternal from below him or on the same level as him.
In the final battle, the Guide says,
Be ready to revive fallen comrades at any moment, but go on the defensive if a teammate dies, striving to survive until your fallen comrade(s) respawn.
When a squad member goes down, it stays down for a minute. So, lose three squad members, you are fighting the Warden Eternal for one minute.
I played multiplayer today. Notes on the multiplayer:
1. Some players have resorted to grenade spamming.
2. Spawn points can be terrible. I was added to games in progress. I would spawn in an active battle zone and get killed within seconds. This happened twice in Warzone.
3. I have acquired through REQ packs a Scorpion and a Banshee. However, I am not able to use these vehicles in Warzone. I can use the Mongoose. I have thirteen Mongooses in my REQ collection. Why would I need a Mongoose in Warzone? This is a vehicle with no offensive capacity and the maps are not large.