[thejeff]

He should have been able to cast Flaming Arrows.
There have been other reports of problems with Flaming Arrows in particular. I think one suggestion is that he won't cast if none of the archers are in range yet.

You turn on debug by passing a "-d" (or some number of d's. The more the more info) to dom3. I use Linux, so I just run it from the command line. On Windows you can do it from Start->Run, or by editing the properties of a shortcut. The debug output goes to a file called log.txt.
It won't interfere with a MP game. You can just watch the battle and it will log the debug info as it recalculates what happens.