Woah, that's amazing Sydery! Unfortunately our demo was not designed to handle that level of dedication, so you can't defeat the Messiah even if you could get it down to 0 health, sorry! That's why you'll have to wait for the full game ;)
Otherwise, the first boss (Harbinger) in the tutorial level is totally beatable! The story will advance the same but you'll get some nice experience points to play with.
Hope that helps!