Keep your opponent moving towards self-preservation.

     Cut off all of your opponent's supplies. Everyone needs proper nutrition to function correctly. Destroy your opponent's water supply, perhaps by busting his drains- this is one of the most important things you can do. A person can live about 40 days without food as long as they have water, but no one can last two weeks without water. If there are pipes leading to your opponent's home, break them. Even if your opponent has them rebuilt or gets water from another source, they will need to pay extra to get those things done. That right there is money and effort that is not going towards fighting you.
     If you can break in to your opponent's home and rummage or cause mayhem, your opponent will spend much more time and effort fixing what you have destroyed then the amount of time and effort you spent causing the mayhem that you did. It is much easier to break things than to build things.
     Start bad rumors about your opponent to both of your allies. Word of the rumors will get around to your opponent, probably sooner than later, and the rumor will dismay your opponent and cause them to loose focus.
     Poison his foods and water source.

     This is what I mean when I say, "Keep your opponent moving towards self-preservation." Your opponent's constant defense will wear them down. If destroying your opponent mentally is your goal, the immediate effects fulfill your desire. If destroying your opponent physically is your goal, constant wear and tear of your opponent will cause them to lose their desire to fight and will make it much easier for you to fight them in battle.