I've seen a few people have a hard time with that last wave in particular, so I may have to adjust the difficulty of that wave. I may have to adjust the written range of units because it includes the spot they're standing on. The speed option is definitely being worked on. Leena is actually stronger later in the game, including the extra map you can unlock by beating the school gates. But you're right that her cost should probably be adjusted.
All great suggestions! Thank you!