Semi-on-the-fly remarks:

1. Game page: I love the context you chose for explaining why the opponent comes back! XD Sounds like an ironic take on the usual respawn mechanic.
2. I see on the game page that you explicitly mention ZQSD; thank you for layout independency! And from this and already from your friend’s pseudonyms and yours, I gathered you might be French. Moi aussi, en fait. :)
3. Ah, you put a start window that lets you select the layout-style; good, nevertheless, it is probably possible to program the keys position-wise (ie whatever the keyboard layout is), as it is in several video game APIs. This is even simpler and general.
4. Some minor English mistakes, for example: a hero comes, This hero has. (Just saying to help!)
5. Sounds would help with getting the action, but I guess you were short on time. jsfxr and Freesound.org are your friends! ;)
6. I like that your represented the sword’s swing (with a red curve), but the sword feel is a bit hard to get, especially that of the enemy; maybe showing the sword, or making the red swing flash, and adding speed to it, would help? Also, the collision area for the sword/hit is not obvious.
7. The health bars are clear.
8. I notice the boss is constantly in a walking animation; maybe he could stop when not walking.
9. I see XP; is this the hero’s (which would be logical) or ours? X) I think it is the hero’s, which is funny once again.
10. I think some representation (such as a blue magic bar, below the green health bar?) of the fireball ‘reloading’ process would help, because I often wondered when I could open fire or not.
11. You can block the enemy behind a pillard and hit him safely with your sword.
    Suggestion to improve AI: maybe make the hero move in a random direction for some time when he collides? Or even look for pathfinding Python libraries out there. (Or program your own if you want. :))
12. What does the white increasing time counter for? Just to know how much time defeating the hero took us?

Conclusion: I love the context/setting you chose for the action. I think you could help feeling the blows (especially the sword), and also introduce more variety using randomness for the hero’s AI, and also changing his abilities and behaviour depending on his level. I think this is a good idea that would be more thrilling with a more varied challenge and stronger gamefeel: the game does not lack that many things at the moment! :)