Boss fights usually devolve into fighting the control lag, and having it walk you straight into the opponent. Precision positioning and good reflexes are of no use because of the lag. It's a fight about lucky shots and stocking potions.
At a guess, the boss & monster routine has a memory leak and the flash engine garbage collection behaves poorly with this, causing the lag. The author suggests saving and reloading to fix the lag, but this "fix" only lasts a few seconds, a minute at best. It also restarts battles in while losing the spent health potions.
I'd give it a 5 stars if the author had not given up on fixing the bugs. As is, it's hardly playable.