You can customize your Doom guy character with several different skins in Doom: Eternal. You can show it off in the Battle Mode, the game’s multiplayer game, or see it in the game’s primary campaign single-player mode. A difficult skin to unlock in the game is the Phobos skin, which you can receive by playing through the game’s campaign, but you need to play it in a specific way.
The only way to unlock the Phobos skin is to complete the milestone Running Up the High Score. You can find it in the Milestones menu on the pause menu. To achieve this, you need to beat Doom: Eternal on the Extra Life Mode with at least 10 extra days tied to your save file. How do you do this?
The best way to go about this is by starting a new save file, going to the Extra Life Mode, and choosing the difficulty I’m Too Young to Die. This is the lowest difficulty in the game, making it extremely easy to fight the waves of demons coming after you and taking on the toughest challenges in the game. The Running Up the High Score milestone does not specify you need to complete Doom: Eternal on a specific difficulty, giving you the freedom to choose how you want to play.
For those wondering, Extra Life Mode introduces permadeath to the game. If you die in this mode, and you have no extra lives available on your character, you start the campaign all over again from the beginning. It doesn’t matter where you were or what checkpoint you were on at the time of the death. You start over from the beginning of the campaign and have to work your way through it one more time.
Unlocking the Phobos skin may take you quite a bit of time. You probably don’t want to do this for your first playthrough. But if you do, good luck attempting to complete it. Extra life mode is an old school approach to a modern game.