Thanks. For the first attemps it's really difficult I know, but once you figured out that you have to switch to a zombie with gun and you have to turn around the player to dodge the bullets ans shoot him, it gets very easy.
Regarding the ui, I'm sorry, I didn't put any icons to clarify what the bars are for (they are the cooldowns for explosion and heal, and the score for boss bars).
I will make an update with all those things fixed