In addition to Focus Target, you can predict High Voltage by watching for Piercing Laser. He'll always move back to his primary hate target and perform High Voltage afterwards without fail. It's part of what makes it easy to silence. If you're used to timing Esuna/Leeches for WARs who use Berserk, then getting Esuna off on yourself or another healer prior would be just another trick to add to your book.