We found it very entertaining and a very interesting idea, since if you get the operations right, you’re rewarded, but if you miss all of them, you still deal some damage.
As feedback, we also think it would be good to avoid having to click on the text box, and we believe the time to answer is a bit short. One idea could be to have several tiers: for example, if you answer in less than 2 seconds, the damage multiplier is x5; if it’s between 2 and 4 seconds, x3; and if it’s between 4 and 6 seconds, x2. This could be indicated with a color on the bar that shows the remaining time to answer.
Another suggestion would be to show a choice when leveling up (more health, more damage, etc.), since right now the fact that you’ve leveled up goes a bit unnoticed.