Demonic Trooper

The Demonic Trooper is a new monster introduced in The Ancient Gods - Part Two, the second major campaign extension to Doom Eternal. They are the warrior caste of Immora, defending it and its denizens from external threats as they await the return of their leader Davoth; when he does return, they stand in defense of the city as the Doom Slayer and the Night Sentinels invade. Unlike the rest of Hell's denizens, Demonic Troopers retain their original forms through regular infusions of Hell Energy, also giving them a form of biological immortality; unfortunately for them, this does precisely nothing to stop them from being killed in battle.

Codex
''Immora’s warrior caste defends the ancient city, keeping watch over the walls since the betrayal of their creator millennia ago. The imperial guard of Davoth remains an honored position amongst the citizens of Immora, wielding armor and weapons forged in the fires of hate and suffering. Their loyalty and devotion to Davoth is unwavering, having waited eons for the return of their rightful leader.''

In-game
Despite wearing full armor and their reputation as the personal guard of Immora, Demonic Troopers are among the weakest enemies in the entire game. Their energy shots are slow and deal little damage, and they can be killed by as little as a single bullet by the Heavy Cannon or a single uncharged Blood Punch; even the Flame Belch and Meat Hook will make them explode on contact. However, they cannot be chainsawed or glory killed, likely due to lacking any real death animations - to compensate, they will always drop some ammunition when killed.

Demonic Troopers can be taken out in any number of ways, but the most effective way to deal with them is to use the Plasma Rifle's Microwave Beam mod; the explosion will kill any other Troopers around them and stagger other nearby foes. It's best to use only one volt of Plasma for this; any more will be wasted on frying their nonexistent death "animation."

Trivia

 * The Demonic Trooper's armor is based on one of the player's options from the multiplayer mode of Doom (2016); similarly, their rifle appears to be the Reaper. Their general appearance and status as elite warriors who nonetheless get slaughtered en masse gives them similarities to the Elite Guards of the UAC's Mars facility in the same game.
 * Due to their use of Hell Energy to prevent their bodies and minds from degrading like the rest of Hell's residents, Demonic Troopers are probably the closest equivalent to basic human enemies in the entire bestiary of Doom Eternal.