I think the atmosphere of the opening is pretty good, running towards the monster and seeing how it towers over you and the camera pans upwards to center on it sells this abomination's aura quite well. I don't know about vertigo, but I like how it does make me feel miniscule and about to attempt a giant killing.
Could call even call Tiago João instead, the spiraling monster being what will be the beanstalk that leads you to a world of giants is... a setting, for sure. (Then I could see the vertigo coming in, as you climb the beanstalk and look back to the city you were in until just now.)
Is the crystal on its back a detail or a weak point? I kept trying to attack it thinking i'd deal more damage, since you deal so little even at max charge. I like the system, but it isn't well polished enough to be truly enjoyable. In a way, I think I'd rather have two dash options, or simply a dash and a backdash, one that ATK+DEF- and one that ATK- DEF+ to make it less about going to the boss, running for a while around it, returning to it. This way, you'd also allow yourself to design bosses which aren't nailed to the center of the battlefield due to your main mechanic.