I'm always a fan of environment changes being out of control (the lego part made me chuckle). What a great idea and great execution! I really like the character designs and the different "rackets" they use, I like how spawning monsters constantly keep you on your feet and I like the bar at the bottom of the screen and how it indicates when the arena is going to change.
My only comment would be that the controls are a bit clunky (particularly the dashing and charging up an attack). I found myself holding the left click hoping to charge a shot, but only in game did I realize you had to hold still in order to make a shot (which makes it more crucial that you land in the same space as the ball, ie. dashing to get there).
Overall this is an excellent entry! And I really like this game (I would play a full version of this in the future) :)