Duke fights our heroes with a vehicle, but never directly.
Only an impersonator of duke is fought. The real one ends up killed shortly after his ringer is defeated.
Duke Darrows doesn't have any fighting skills and wouldn't stand much of a chance against the heroes, so he avoids a direct confrontation at all costs.
Duke is, in fact, on what would end up being the hero's side, as well as Evulz for once. Halfway through, shortly before you catch up with Duke, a major reveal is shown, changing sides dramatically with the heroes actually agreeing with Evuls, and by extension Duke, removing a need to fight.
Duke isn't really a person. He's actually a disguise designed to be able to be put on by many people of the same height and weight. You don't find this out until later, when you think you may finally be able to face him.
Duke is killed by the real Big Bad before the heroes can get to him.
Duke Darrows is a badass who could dismantle the heroes with ease, so they must avoid fighting him at all costs.
Every battle in the game is against Duke.
The heroes chase him down and take him on.
The credits roll, and the boss fight with Duke takes place afterwards.
However, after the battle ends, the Duke they fought is revealed to a body double minion.
Parodied: Duke Darrows looks like a total Bad Ass but at the final boss fight he simply leaves and you fight a minor mook from the beginning of the game that Took a Level in Badass.
Zig Zagged: There are several routes in the game. One of them you end up on the same side as Darrows, another he looks like he'll get used as a diversion but getsexecuted for letting the heroes reach the final sanctum, and finally in one route.
Averted: All major antagonists are faced in battle.
The creators want to avoid an Anticlimax Boss, so they keep the player from ever fighting the Duke.
It is difficult to conceive how a fight with Duke would play out.
It's a licensed game where the characters in the original work didn't fight the Big Bad directly, and the studio wanted to be true to the source. (Or just couldn't figure out how to add the scene.)
Technical limitations make the boss impractical despite being so integral to the story.
Lampshaded: "Damn it when will we get to face him?!" "I don't think we will."
Invoked: Duke Darrows makes sure to operate remotely so he can never be accessed.
Exploited: Duke Darrows can't be fought and he knows it. He never wields any weapons nor and wears no armor. Instead he shows up in outfits that would be lethally impractical for anyone else and taunts the heroes unceasingly.