Most of you probably already know this but as I couldn't find any post on this I thought I would post it. I did see someone posting a Hydra method in the A4 warden post, but it doesn't work very well.
An easy way to kill most of the wardens A1-3, is by standing off screen and use skill such as Orb (Wizard) that has no range. For example for the A2 warden you will see his shielding, stand far enough away that you can barely see it, and if you do agro it just run away they will disengage pretty fast, then shoot at it off screen. In A2 warden you will even get electric sparks off indicating it is working. Because the way they behave right now, they would not chase after you unlike Elites. I assume this will be fixed pretty soon, so take advantage of this and get a lot of keys!