@Verelil -- Hello. This sounds similar to the issue some players were concerned about in the ice caves -- there seems to be a zombie stuck off the screen that nothing can hit, but that prevents the level from completing. I can imagine that this would be more frustrating than fun! Well, here's the deal -- this forum was put together by EA, but it is a forum for players to help other players, so there isn't much we can do about glitches except direct you toward EA/PopCap support. From one of the sticky tabs above, here are the links they provide.
For Plants Vs. Zombies 2 we have these direct links for iOS and Android users:
iOS: http://support.popcap.com/pvz2
Android: http://support.popcap.com/pvz2-android
You can also contact Customer Support from within the game on iOS and Android by tapping Help in the game’s Main Menu, then filling out the form that appears in a browser window.
If this helps resolve your issue, please click accept solution as this may help others with the same issue to find a resolution as well. XP is appreciated but not required.