Many difficult people behave the way they do because they feel underappreciated. Whenever you have the chance to build up a difficult person in conversation with a third party, seize the opportunity, even if he is an annoyance to you.
If others say something good about him, pass on the compliment. The difficult person will appreciate hearing the comment and will feel positively toward the person who made it. Whether people do their best or worst work for someone often depends on their relationship with that person.
Creating positive relationships is one way you can attempt to raise the level of someone's work. The third-party compliment technique also forces you to focus on the person's strengths rather than weaknesses.