I agree, it's obnoxious as hell, which is why I'm pretty sure no one in my FC ever wants me to tank ... ever. I like to randomly jump around and cleave everyone.

That aside, the issue you're seeing is that the spellcasts are extremely short and the mob's LOS is a full 180 degree semicircle in front of them. It's unlikely to break their LOS them on normal reaction; you need to begin moving to LOS before they actually cast the spell. The easiest way to get familiar / observe this mechanic and timing is to find a random sprite in the general zones. Sprites chain cast. Off the top of my head water sprites at Chimera and the ice sprite in one of the Mor Dhona leves.

I'm actually trying to convince our FC tank to learn the walk-through method rather than simply circling around them, because it's &&&&**#%%%%%%%%@#$$$@%ing annoying to get my Heavy Thrust on the target when he's doing it. If you walk through the mob repeatedly, you can pause for about a half second every time.