When he marches on into my final level, eager to beat me down and slaughter my hordes of minions, I'm going to have the following obstacles awaiting him:
*My diplomat, who will offer peace terms to him, rather generous ones that offer him considerable blood money to make up for my evil deeds that affected him, up to and including offers to revive dead loved ones and rebuild that nice villa of his my men so callously burnt down.
*After he kills the diplomat on most playthroughs, up next will be the love interest's mother, who I will have spent the game quietly romancing and just married about three days before he showed up at my doorstep in a tasteful private ceremony. She'll plead for my life to be spared and for all this bloodshed to end, because naturally over the course of the courtship I would have been nothing but the tragic, misunderstood gentleman (to her, anyway). Even if he kills her to move on, that's going to be a hard one to explain to the love interest, doubly so if she's standing there with the hero at the time.
*If he gets past that, I'll have in place a locking mechanism for my inner chambers designed like a malicious mixture of Bejeweled, FarmVille, and Tetris, and make the indicator saying 'You can stop now, it's unlocked' so small and passive the hero might not notice for hours, assuming he even figures it out.
*Lastly, before my throne room proper would be the obligatory old mentor turned heel. Except in this case, it would be because I'm the only one who can cure his sickly daughter, and as such the old mentor has to defend me or lose the only family he has left. If the hero didn't totally screw the pooch with his love interest before with the whole mother business, then killing the mentor and condemning his innocent daughter either to be alone in the world if he fails, or to waste away and die if he succeeds in his vengence-driven desires, should pretty much kill that romance deader than 18th-century disco. Either way, he likely goes on alone from here.
*Finally, after all of that, he'll arrive in my throne room, the den of all my evil deeds, where he will have to face me in... a dance contest. Yes, that's right. The final battle is a dance contest. Why? Because that's how it's programmed, bucko. And if you win, I just leave and conceed the victory, vowing to come back in some vaguely conceived of sequel. No final blood-splattered win for the hero or any force controlling him, no justifying kill to make up for the passed-up chances and dead innocents, just him alone in my former throne room with everyone's blood on his blade but mine. Roll credits, curses out the development team, and throw your controller through the screen, game-boys; you won the battle but I won the damn war.
...or, y'know, just send some ninjas after him or something. Either/or.