A couple of weeks ago myself and three others managed to get to I think level 26 or 27 at Tumbleweed. All of us were above level 50 so that's probably why we did so well.
I also managed to get a 70 kill-streak in this game, no word of a lie. I think it was around level 15, and I threw my holy water at exactly the right time at the piece of land inbetween the town and the church where all of the zombies spawn. 69 zombies died in the blast and I shot dead the last one.
I just wish there was an achievement for something like this - a 40 kill streak is not enough!:annoyed: