I also feel that The Blimp is too weak. I think waves 3-9 should stay as they are. However, the last wave would like to have a boss who has to create a more difficult to explode "Super Blinp" etc. and inflate it more.
Even harder difficulty may be interesting. I think it will be quite difficult even if the enemy does not explode unless it is inflated (improved the difficulty of obtaining items). (In hard mode, I was able to clear it continuously.)