Absolutely fantastic game. Great art, the animations are smooth, and I am really looking forward to the full release when I can recommend it to my friends.
But yeah the Leslie fight at the end of stage 4 needs some tweaking to the sword attack, there is absolutely no indication on how to either block or dodge it. It's not consistent with the other attacks either: you can't shoot to stop the ground slam. At first I tried jumping over it as Leslie does flash like that would be the timing point for that, and it did say in the tutorial image that you jump to avoid ground attacks.
Works perfectly fine on Linux via Wine :)