Subduing Private Hendel
At the end of The Missing Diplomat quest chain, you must subdue Private Hendel. He is guarded by two level 35 accomplices. Immediately after you accept Hendel's challenge to fight, his accomplices will attack you. There is a trick to defeating Hendel and his accomplices: immediately after accepting Hendel's challenge run to the edge of the entrance ramp to nearby North Point tower. It's only a few steps away. Hendel's accomplices will follow you to the edge of the tower -- make sure you don't outrun them. When you get to the entrance ramp of the tower, guards from North Point Tower will come down the ramp to assist you. Once they've taken care of the accomplices, you can go back for Hendel.
Another strategy is to focus all of your attacks on Hendel and ignore his guards, because once Hendel's HP is low enough, Lady Jaina Proudmoore and Archmage Tervosh appear on the scene to finish the job. The guards run away at first sight of them.
If you fail to subdue Hendel, it will at first appear that you're not given a second chance to do so. Right-clicking on Hendel no longer triggers his challenge. To fix this, abandon The Missing Diplomat quest, then right-click on Hendel to re-create the quest.