This is a really clever and unique mechanic. I had a blast with the tutorial boss and wondered if the ability wouldn't be overpowered ... then the next boss shut me right up! Geez, that was a huge difficulty spike. I think if you'd given another 15-30 frames between the attacks there would be clean enough opening to lay the wall and launch it, but there would still be plenty of challenge.
I ended up cheesing the boss. I discovered I could just create the walls on him and damage him without needing to launch them. I think you could probably prevent that exploit with another variable and tile detection ahead. Because the core mechanic is really creative and fun to pull off. I just needed a little more time to make it happen.
Overall, this was really well polished and one of the most impressive entries I've played so far. Great work.