So it might seem like an easy boss fight here, which it is once you know what you need to do, but it took me a while to get a good strategy down. The Warden's weak spot is his back. So what you need to do is go up the ramps to the top. Order your squad to put fire on the Warden to distract him. You should be able to get at least one good set of hits on him. If you have a set of grenades you'll also be able to reign down another critical hit on him. If you don't have grenades there should be some behind you in the gun caches. On top there is an incendiary cannon. The Warden can still shoot you with laser vision so make sure to just keep peeking. IN the meanwhile, until you can get a good shot, try to pick off all the minions and keep your squad alive. Hope once you get a good angle, another 5 to 6 shots with the cannon should take him down. The main thing is try and keep your squad alive. Keep telling to move if the Warden is getting too close.
Playing this is co-op mode is a breeze since you can just surround him, but on solo it's hard to control your troops. It's a joke how easy the boss fight is on Normal, but it's a different story on heroic and legendary.