Obtain the Spoon (or Golden Spork) Melee Weapon. After doing so, go to the Warden's House and melee the wall across from the zombie barrier and Titan Wallbuy with the Silver Spoon / Golden Spork to scratch the wallpaper. (Optimally, you could melee the wall with the Silver Spoon then focus on upgrading it to the Golden Spork while waiting for Round 17 if you don't start early using Monkey Bombs, see below.)
Make the Warden use his Slam attack in front of the now scratched wall, either naturally starting on Round 17 or sooner by having him interact with a Monkey Bomb in front of the wall. (You can obtain a Monkey Bombs either through the Mystery Box or completing the Monkey Bomb Statue ritual in Cell Block, but they will replace your Hell's Retriever.)
Interact with the electronic switch inside the Warden's Ritual Room and watch as the Warden's corpse becomes revealed, dead on an electric chair.
Take the small, glowing red orb on the table to the left of the Warden's body.