The warden is the big heavy dude that spawns when you first activate the second power switch in the actual prison. He will also spawn every now and then on higher rounds.
In order to get this achievement you need to have the power turned on and then either go to the docks which are located right next to the power switch or go to the highest floor in the cell area where you will have to go into the gondola and activate it (or call it first if its not at your location). It costs 750 points to use the gondola.
Now kill most of the zombies in your round first and then lure the warden into the gondola. When inside activate it and complete the ride with the warden inside. You may go down during the process although it is possible to outrun his attacks by circling him in the small space you have. If you are playing on solo you should simply revive yourself automaticly by the end of the ride if you havent gone down more than twice in this game.
Playing on casual also makes this much easier. I would not recommend taking bots with you for this.