This is really great! Extremely impressive that this is 300kb, i have absolutely no idea how you've done that but it shows in how smooth the game is.
The mechanics feel good, Everything i love about dark souls is there. The boss has a clear attack pattern which can be learned, there's a few delays and stutters in there to throw you off. The second phase with the fire sword adds another layer of challenge. Really top work.
Having the choice between the two cameras, and to switch between visual styles, is a fun addition which makes the game feel really different. for me personally, i love the over the shoulder view, but the isometric view makes it feel very runescape like.
Overall, really top work. You've captured the dark souls vibe, added your own fun twist to it and created a really mechanically solid game!