I think the boss needs a better indicator for when it's going to actually swing its sword. Maybe the boss could also have 1 or 2 more kinds of attacks that can be parried, the parrying is quite hard at first but becomes very easy since the boss only has one attack.
Also spamming the attack button instead of parrying makes the boss too easy as well, perhaps have a short delay on the player's basic attacks?
Cool game, I can see the potential of it.