Part of this script is redundant - the magic user bit, but the thief part ensures that only a thief of 11th level or higher can ready this item.
If the monsters won't use this item, it's because the AI thinks that the monster has a better attack. The AI_Script.BLK is the script that contols this, so Nol or Paul are the people who have the expertise to explain what is happening.
As far as duels and/or solo combat, search the forum for a proof oc concept mini-mod I did, I believe I called it "solo combat".
In a nutshell, it uses a script that sets the status of every party member except the selected one to a status of TEMPGONE.