I have finally defeated the first of many warden eternal in the Halo 5 guardians campaign. It was getting to the point where I thought it was impossible without a coop partner so I looked up a guide. I never seen so much useless -Yoink- on a game walk through before. People were saying you could get your teammates to distract him but that clearly is not the case because even if I deal no damage he is locked onto me. The scattershot dealt no damage when shot at his back, only the incineration cannon proved to be effective.
Essentially what you have to do is grab the incineration cannon and head down to the other side of the map (the opposite side to where he spawns). Then you run around the pillars and force your teammates stay down there with you. Keep running round until you can get a shot at his back, reviving teammates as you go. If all else fails try to get him to lunge of the side.