Wardens look like alien version of the Hulk and as you progress through the main storyline you will fight them to gain elements for your superpowers.There is a circle around your map that is split into 6 parts and you have to fill up 5 of them by killing aliens and the warden appears.
Method 1: (Recommended)There is also a place in Steelport that is an alien controlled area that is east of Downtown and South of New Colvin. When you get to this area just kill aliens untill you get a warden kill the warden and repeat. If your having trouble i made a video for it.
Instead camp out in front of a gun store and kill aliens untill a warden comes. Wardens come after you get your Alien Notoriety to a certain level.
I was in front of a gun store so i could complete all of my gun challenges.
Hopes this helps pm me or comment if you have any questions or anything to add to the solution. If you give me a negative vote please comment or pm why so i can fix it.