Interesting idea you've got there! The concept is great, the system is complex enough but still manageable. It takes me a while to understand what's going on, but after then that it's smooth sailing. Good job!
My little criticism would be the UI could do a bit more with either animations or effects, so that the effects can be seen more clearly. Putting the potion on the top of the screen would also be a minus since you are constantly putting ingredient into the potions, you are really moving the mouse a lot. I'd also say that only four ingredients don't really leave much choice to the player, as in later levels what I usually do is just dump everything into the defense potion, and the remaining into attack. That makes using effect much more punishing yet without much reward. And it also drags out the battle a bit too.
But overall, I'd say this is a nicely built game. Well done!