We only get to see it happen at the beginning of the video, but it seems that Valentine hits Double with burst, and Temple Tyrant's SA seems to activate and grant immunity. One thing to note is that we don't see the immunity buff.
It makes sense "lore" wise as Morpha was a water based boss in Ocarina of Time that could only be damaged in a certain area.
Other things we see in the video include 2 "new" skill effects, Cilia Slide applies a slow (more like CHILLia slide amirite) and Bogus Buzzard applies heal block.
We also get a glimpse of her blockbuster Beast of Gehenna. If it behaves like the PC version, the "trap" will disappear when she takes damage, gets thrown or places another one down. In the PC version however, you can jump over it, which isn't really possible in SGM.
EDIT: Confirmed on discord that BoG can be blocked and has a time limit before it goes away.