Well, Halo 5: Guardians has been out for a week now, and since the blanket ban on spoilers has now expired, let's have a discussion on defeating the newest antagonist of the series: the Warden Eternal.
So, first things first. The Warden has three main attacks: a heavy melee charge, a yellow, fast-moving projectile, and a purple, slow-moving projectile.
The heavy melee charge is interesting in that the blade doesn't actually have to hit you in order to inflict damage: instead, you just need to make contact with any part of the Warden's body while the animation is in progress, meaning that even trying to move to his left side doesn't guarantee your safety. However, it is possible to dodge this attack with the thruster pack, even if it does seem to be a little luck-based at times. Your best bet is to try and use the geometry of the level to your advantage, moving around an obstruction rather than just trying to evade in the open.
The warden's first ranged attack can be cast even while he's moving, meaning that you won't be able to just try and run away from him if he's chasing you down with his melee. However, it only ever moves in a straight line and while the projectile moves extremely quickly it ultimately does have a travel time, meaning that with quick reflexes and a thruster pack, it's possible to dodge it.
Lastly, the warden can summon a large purple orb, which he then casts at the player. Although quite slow-moving, this attack does home in on the player, and so is almost impossible to dodge conventionally: worse, it also deals a huge amount of AoE damage which, unlike most explosions, is capable of injuring players even through cover. Since the projectile always tries to move directly toward the player, regardless of sightlines, it's possible to avoid this by simply putting something solid between yourself and it and standing well back, as it will harmlessly impact the obstruction instead of trying to go around it.
Next, the Warden has an extremely large pool of health, and is only susceptible to damage in certain areas - most prominently, the large glowing region on his back, as your AI teammates will be quick to let you know. What they do not let you know, however, is that the good old fashioned headshot still works! Though he cannot be instantly killed by headshots as with most enemies, shooting him in the head does deal damage (though, I believe, at a reduced amount) and can potentially stagger him, allowing you to get around to his back and attack his main weak spot directly. And yes, it is possible to assassinate the Warden with a melee, although this can only be done once he has already been seriously injured.
So, with all the above in mind - what are your strategies for engaging the new Warden bosses when they appear throughout the campaign?