I haven't tested every situation, but assault slash seems to hit separately from the player

The enemy's location can change: hit The player dies before it lands: hit The player is frozen before it lands: hit The enemy can be using defense/counter: hit Windmill and other invincibility frames: untested

It appears that the attack and the player are completely separate (Similar to fireball). The player can be knocked back and the attack will still land though I've only seen it happen once and have yet to get the timing down enough to test/verify it.

I've tried Windmilling in PvP against a delayed AS, and I got past it. :P