What's your strategy for using the silver pendant?

I finally grabbed this handy little toy, and am loving how it neutralizes the Oolacile mages. The timing can be a bit tricky, though, since the anti-magic field is very brief. Has anyone found an optimal time to hit it when facing a mage? In the short time I played with it this morning, I couldn't really tell at what point the shield kicked in...I was more or less just doing it as the mage went into casting motion. Is that about right?

Use it against the final boss, otherwise, don't use it.