Been a minute since I tanked that one, but if it's a big glowing red bar for the cast, then you'll interrupt instead of stun. Interrupt is Interject and stun is Low Blow. Low Blow will work on most normal non-red cast bars.
Don't read guides for story content. The only reason it's a thing for Extremes and Savage and stuff is that those fights are harder versions of the originals so you already have seen the mechanics in the original. But you'll still find completely blind PFs because people like to figure things out. You're a ways off from having to worry about that, so just enjoy the MSQ for now.