That was pretty intense. I didn't notice the changing time signature until someone else pointed it out, but I think that's because it all ties together really nicely. I'll be honest, I think I like the second half of it more because it's not as intense and it's got some nice guitar parts in it, but I think for a video game boss where you want things to get weird and chaotic I think it's really fitting. I think there were a couple of Paper Mario songs that kind of sounded like this but I'll have to go back and look to see if I can remember which ones they are, lol
But, yeah, it all fits really well together. I don't think the transition is as harsh as you would have me believe, like if it's for a video game soundtrack then I think it's a good way to shift the tone depending on what's happening in the boss battle