(I am now in a rush to test as many games as possible to help, so please do not mind the cursory reviewing style.)
nice scenery!
Wow, super music!! :o Impressed you did this in so little time (according to the game jam page)
I like the black cube’s movement, its animation for flattening and shooting a laser
I think you should definitely clarify what you mean by "within 0.5 seconds": at first, I thought you meant when the animation showed our cube attacked, but actually, you mean AFTER PRESSING ON ANOTHER ACTION!
Once I understood this, it became very easy.
beat the big black boss cube — and the game!
I think it would be more interesting if you had to press block at the right moment when you get a sudden attack from the boss; otherwise, there is no surprise. I like all the scenery, visuals, music you managed to put in place in a game made at the ‘last minute’! As for the constraint, as you say, you did not really comply to it.