How to Kill the Warden
The best way to kill the Warden is to Freeze him using the Cryo Gun. It is found in a safe room or as an enemy drop. Find the Cryo Gun and use it against not only Warden but other big enemies. Once frozen, attack the head to deal as much damage as possible. Avoid his melee attacks as they are one-shot-kills. When avoid Cryo freeze he will attack so the best way is to use the Boom shot.