The plain ol' defense buff that just reduces damage comes from her normal overdrive.

The giant damage cut shield that wastes 5 attacks comes from the first of her two special triggers. If you see her raging (or you yourself knock her into raging), then the first action she takes is that hit all attack that lowers your water resist and procs her shield.
Someone who has already connected to the fight, was around to see her enter raging mode, and then takes a turn? That shield gets procced.
Someone connects to the fight while she is in raging mode and then takes a turn? That shield gets procced.