Thanks for the screenshot. I can absolutely fix this. Can you tell me what display resolution your monitor is?
When you collect 100 energy you can use a special move which will lower the health of all zombies to 0 and raise your health to 100. From there they can all be killed in one hit.
As for difficulty, yes there are stronger zombies, they can appear at lower levels but will appear more frequently at higher levels.
Thanks for the comments on the menus. I will need to fix those up. They are still a work in progress.