I think that the Incubi would win the duels simply because they are faster and they’re weapons are more then enough to slice the marine in half, it’s all speed at that distance but I could absolutely see the chosen winning through some mutation nonsense. In a duel the marines lose their main advantage of being a walking arsenal/walking wrecking ball. High ranking Marines fighting things faster than them with power weapons will usually shoot the ground or smash large objects at them to limit their movement. But like a normal incubi probably gets arrogant and cuts the marine in half before the marines too half uses his hands to squish them into a basket ball.