Playing through the campaign solo on legendary, i've noticed a few strategies to beating the warden. when playing solo especially legendary solo, using your Ais is an ideal strategy. Aggro the Warden and keep him focused on you while constantly marking the Warden as the priority target for the Ais. Since every attack the Warden uses is a one hit KO on legendary its best for you to stay at a medium distance, but close enough that he wants to follow you instead of your Ais. this strategy works well on the first encounter with lockes team and the first encounter with blue team. During the battle with two wardens, aggro only one warden and bring him out to the open area (not the grassy area) in the back which lead to each warden spawn. Kill one and move on to the next.
Lastly the battle with three wardens. Almost every attempt I made during this battle ended in tears. I had to learn the patterns and the way the battle worked before i could win.
Firstly in that battle each warden is restricted to his own zone.
I chose the warden that stationed himself on the left of the entrance. Aggro him and use the same AI attack routine. If your Ai's aggro the warden immediately shoot his glowing back to get his attention back on you.
Make sure to limit your time in the open area because the other wardens will attack you and will kill you. Stay alert when you know another warden has a visual on you. He cant charge you with his sword because of his restrictions, but he will use his eye beam and the purple stalker ball of death.
Once you have defeated one Warden, another will come to engage you regardless of your location. Also each time you kill a warden in this battle it will trigger a checkpoint. Once you have defeated the second warden, the third warden will attack and 4 large focus turrets will spawn. the four turrets total cover the entire field so they are top priority, especially on legendary because they can instantly kill you. Make sure the warden is focused on you and order your ais to kill one of the turrets on one side. then do the same for the second one but provide assistance. stay on that one side as the only enemy you will have to worry about is the warden. I personally trained him around the ramp. this strategy also comes with risk because in some areas you are exposed to the other sides focus turrets. i used this strategy if all my ais had been killed and if i had to wait for them to respawn.