There's a couple ways I can think of.
Take away that one person he interacts with. 24/7 in the most horrific, mind-screwing, gruesome way that the hero can conceive. Then prod him and use the plot to keep reminding him of the fact.
Show him multiple failures at the same time, in the right places.
If an enemy pulls off a plan, have the hero find out what it is, prepare for it, then still fail. Better yet, have the villain use the hero's failure to speed his own plot. A quote I heard once is this:
Quote:
"In any strategy game, there are ultimately only two moves one can make. React to your opponent's strategy, or advance your own."
On one of my stories I dabble in now, I'm gonna kill off the first person my character ever met in front of his eyes in a bloody mess. Despite the over-extreme-ness of this plan, sometimes an extreme is what pushes them over the edge.
If you don't already have such a character, have a deceitful love interest. Hormones, love, attraction, and lies are terribly difficult to tell apart.
If you have the perfect hero, design the perfect villain. Better yet, make a heroic companion who defects, or a perceived good guy that the hero himself can look up to, but with that one flaw.
Ender's Game had such a person. Read about Dink Meeker's fatal flaw according to Hiram Graff. (That's about as non-spoiler as I can get it.)
You could character develop the hero in a certain direction, and then set him in a no-win scenario where whatever he chooses, he has to live with. Have this scenario tie directly into his character development.
The issue I think sometimes happens is if you're trying to specifically break someone, you introduce new aspects that are driven only to drag your hero down and smash him to pieces. It may work, but now you have these new guys you pulled to drag him down.
What do you do with them?
Bleach, the popular Japanese manga I read, has a problem with this sometimes. Tite Kubo, the author, admits that whenever he gets stuck with the plot, he automatically introduces new people in just to push the plot where he wants it to go, ending up with loads and loads of characters.
(It's sort of turned into an advantage, because near the ending of this series, he's been able to take those existing loads of characters and bring them back for a second debut. It's still fueling the plot, but now the audience knows who they are at least. It's also a pleasant surprise for many.)
I think it's better if pre-existing devices, plots, and people who are already in the story are used. People are already more familiar with them, and might accept them better than this one guy who comes from left field as the direct opposition of the hero.
Some links that might (or might not) help.
Proceed at your own risk.http://tvtropes.org/pmwiki/pmwiki.php/Main/TragicHerohttp://tvtropes.org/pmwiki/pmwiki.php/Main/FallenHerohttp://tvtropes.org/pmwiki/pmwiki.php/M ... Derailment (This one is more general)
http://tvtropes.org/pmwiki/pmwiki.php/Main/OCStandInhttp://tvtropes.org/pmwiki/pmwiki.php/M ... lFaceIndexhttp://tvtropes.org/pmwiki/pmwiki.php/M ... tyBusiness(I realize that I keep talking about The Hero. Sorry if that's not who you were asking about, but it seems like the person you're trying to bring down is some sort of Archetypal Heroic character.)