The only difference between Marines fighting Marines and fighting humans is that humans just run away and get killed. Marines fighting Marines are all one-hit kills.
But humans fighting humans is also one hit kills.
Power armor doesn't protect. Transhuman physique doesn't endure. Nameless character don't het wounded. Fire doesn't suppress. Tactics don't apply. Skill doesn't protect from blades.
You're either running straight at the enemy or the enemy is running straight at you.
It's all weightless and impactless.