This is a great start! Your pixel art style is very charming, and the animations are good. The premise is fun, with having only 1 HP. The combat itself is pretty good, but with a few iterations I think it could become really fun. It's not far off, but I do think there are some things that could be improved.
First, I'm not convinced that a shield is the right tool here. It's not really useful at all. Why block when I could be swinging my sword, which creates a barrier in the direction of my attack anyway? I don't seem to get anything out of blocking that I don't with attacking, so I tried to use the shield but never really felt it was necessary. A roll seems far, far more fun and applicable here. It not only makes things much more dynamic, but creates a lot more exciting gameplay moments.
Second, I think the attack timings are a little strange. That's not always a bad thing, but it appears that there is a big delay from hitting the button, to when you actually swing. And it's unclear when I am actually actively attacking, and what the shape of the attack hitbox is relative to the art. It almost seems like once you hold the mouse, you are sort of invulnerable from all sides. Leading me to just stand in place holding attack, waiting for the enemies to slowly file in towards my death blades.
I think the game would be better served with more clear windup, attack, and recovery frames. That is, when you hit the attack button, there is a vulnerable period before your attack is actually active, then a period where you're attacking, and then a vulnerable period where you recover. Even better would be if your attacks done in sequence were more of a combo, rather than a a linear repetitive sequence. Imagine if your first attack was a fairly safe but lower damage swipe. If you hold the attack button down, you then do a different attack, a slower but more powerful stab or something. Then if you keep holding the button down, there is an even slower but even more powerful attack. That kind of risk/reward gameplay at the micro level is what makes combat fun and satisfying, in my opinion.
Third thing, I think the enemy AI design can be improved. It feels right now as though the enemies all sort of just file in, and then eventually do a little swipe at you. For the stakes of the game (being killed in one hit), there should be a much more clear "windup" phase for the enemy attacks. Without that, you can't really make them difficult without making the game impossible. So you get stuck in making the enemies basically trivial. Imagine if the grass monster thing would like start flickering red for a second, then charge at you really quickly. With a roll, you'd have to time your roll to get out of the way, or if you were confident, you could queue up an attack that was times just right to meet him as he charged. Now you have options like, what if while he was charging, he was very vulnerable? Then if you were brave and tried to time an attack to meet his charge, you'd basically one-shot him.
Just throwing some ideas around, but hopefully you can see where I'm going. There's a ton of potential here, and I think it's just going to take a bit of experimentation to reach it.
Oh yeah, one more thing. Because the entire premise of the game is having 1HP, I really think you should show the HP bar, and really make it clear that it only has one point. That is funny and endearing, and drives home the conceit.
Nice work, keep it up on this. Like I said, I believe there is a lot of potential here.