I've finished 19 of the 20 Challenges on Worms for Live Arcade and the final Challenge is driving me crazy. It's super difficult on account of the bunching of the worms and the insanely accurate enemy worms. I don't want to call the game cheap because I believe that a really good player could pull off the shots that the enemy ai does, but not quite with as much regularity. Nevertheless, unlike other games that stop me in my tracks, I don't get mad playing Worms. I take my beating calmly, go upstairs and work for another hour or so, then come back down and try again.
I've read online about people using the girders in the game to basically build themselves a little fort in the upper corner of the area and waiting for Sudden Death to flood the other worms. I know a friend had done this to get past Challenge 13, but ther's something really odd about seeing no score for him for that Challenge on the Leaderboards, yet see them for subsequent Challenges which he was able to pass without having to resort to the cheesey run-and-hide tactic. I'm going to try fighting it out like man-worm a few more times before I give up and try to win by default. So far nobody on my Friends list has made it past Challenge 19 but me, but there's a few closing in.