The Manor Sentry, at (10.5, 11.5), worked for me. No need to mess with the boss. I was level 53, before I tried to get this, so I unequipped armor, but not jewelry, lowering Magic Defence to 3, but the log in Battle chat showed that I dodged every Petrify, yet I learned Petrify all the same, which surprised me. Right-clicking on the Manor Sentry once to start auto-attack, it took a few minutes; nothing else to do really, but watch. Best of luck to you.