To fight/finish off a warden use your blast or stomp abilities to break his shield and then unload on him with your gun of choice. Personally I had to do all my kills in my own game. Something important to note is that if you kill a warden while in some else's game it will NOT count towards your achievement or your challenge it should be noted that in some cases warden kills do count in another players game as long as you're the one to finish it off. So you will need to grind this out by increasing you notoriety to max several times. They also show up as a part of the story to give you new powers, but those are limited and not enough to get what you need for this achievement or the corresponding challenge for killing 30 of them. Also doing flashpoints can possibly spawn a warden.
They also show up if you get you notoriety to the 6th bar as soon as it reaches this point a single warden will show up.
Once you complete one, you have a chance of a warden showing up.
