There is a way to skip the Warden through a glitch in the campaign, but that's for sissies.
Usually the Warden boss battles come with some power weapons around the battlefield. Explosives are always handy as you don't need to hit him in the back with them, just make sure you hit him.
With the single Wardens use your AI teammates (or real ones) and get his back turned to either one of you so you or your teammates can shoot him. I found that letting the AI do their jobs is alot easier than to try to shoot him yourself as the Warden always seems to go for you instead of the AI. Besides that the AI are a pretty good shot, just don't forget to order them to attack him.
Also, don't forget to clear out the minions first, especially on legendary, since these guys can get quite annoying.
When facing multiple Wardens, focus on one at the time. The first time you encounter multiple they're quite far apart so you won't be bothered by the others as much, make sure you're out of sight though as they will still laser/bomb you.
At the final battle where 3 Wardens will come at you in a relatively open arena use the Incineration Cannons that spawn on all walls except for the one you come in at. Use these at one at a time and let your teammates shoot him while he chases after you. The pillars are also a pretty good hideout as he can't seem to get a good shot at you up there.
The last Warden in that arena will spawn in laser turrets which you'll have to deal with first to avoid getting killed. Make your teammates fire at him and distract him while you take down the laser turrets. Wait for the turrest to charge up as their weakpoint will be pointed directly at you during the charge time. Even an legendary you shout be able to bring them down with a Lightrifle before they fire.
This is how I dealt with him and I hope it helps you too!
Good luck, you'll need it