Sorry to hear about your troubles. From my understanding google puts more emphasis on contextual links that point to your site, where as Bing and other search engines will look more at your on page SEO, which looks good from what I can see for zombie games.
The only thing that I see right away is that zombie games is not in your URL, which would help you rank for that key word. The issue you face to me would be either to put more effort into building backlinks to your home page for zombie games, or to create a subdomain or another page with zombie games in the URL:
zombiegames.killthezombies.com or killthezombies.com/zombiegames
Which actually might be over kill I try not to repeat terms in the URL as it looks spammy to me maybe:
games.killthezombies.com or killthezombies.com/games
If you're not showing up in the top 100 this would be the tactic I would choose, but this would be like starting from the beginning and you would have to build links back to this page to really complete with other sites ranking for that keyword.