Zombies map five question?

I have tried every way possible to get crawlers to last more than 30 seconds, i let them hit me, throw the bomb further away, tried on higher levels up to 16, but they all die within 30 seconds every time. everyone keeps telling me ways to do it but they just keep dying. ive seen videos where they last a very long time. does any one else have this problem or know any other ways to keep a zombie to crawl longer so i can get stuff done at the end of the round??