I agree that the spawning needs work lol. As a quick fix because I was running out of time i just gave the player immunity for two seconds at the start of each wave so you wouldn't take unfair damage. ^^
Oh this had some unexpectedly clever puzzles! It took me a while to get through level 6, and later on i didn't notice that the boxes didn't use up platform durability, but i got there in the end. Nicely done! :)