i havent tried them out much yet, but what you could do is use the experience deck instead of the normal skills, ie when reaching a level you only use the experience deck but not the skill on the survivor card, this should rather increase difficulty
Ouch! Guess your right about that. At the moment I am toying with the idea of keeping the Experience cards rules as written but also increasing the zombie dice pool by one dice for every 3 Experience cards. Not sure if that will work but it should be more interesting than our first solution which was to have players drawing cards at random. That didn't work out.
we have also house ruled that wounded cards carry over but a food item can be used to remove a wounded card AFTER the game ends. That played out to be great fun and finally gives a reason to hold onto those canned goods.