Queen of J-Horror

Quite a recent movie with the house haunted by a vengeful spirit clichee we all know. This one sounded interesting to me cuz the spirit is obese and I wondered if there was a reason for that (and there was, which is incredibly heartbreaking).
I liked the pace of the movie until about halfway, when the dementia-plagued (I think) grandma turns into a hippie ghost-repelling-pro, using sheer force of will and tai-chi or smth similar to literally beat the sh*t out of the spirit. It was just so random and totally took me out, given that almost her entire family was just murdered and she could've done something to prevent it, but didn't "transform" until they were dead.
The story of the spirit is unbelievably sad and I couldn't help but feel sympathy for her, but the part where hippie grandma effortlessly gathered her living family for her to murder and then having a boss fight with the spirit after that was just weird. The love interest of the protagonist being alive "inside" the spirit instead of being killed like the family members also seems convenient for the plot.
Overall it's just the usual haunted house with the family trying to get rid of the possession but ironically not using the shaman/religious route but beating the spirit with sheer will and determination. I like the concept of not giving into the fear and despair spirits, ghosts etc would want you to feel, but the random comedic elements feel so misplaced in this otherwise okay plotline