Scrape points from CTF sites
You could use a list comprehension in get_num:

def get_num(string):
    """this function takes a string as input and outputs a list with all the integers in the string"""
    # get the numbers from string
    numbers = ''.join(x if x.isdigit() else ' ' for x in string).split()

    # change to list of ints instead of strings
    return [int(number) for number in numbers]
    # return map(int, numbers)  # Alternative


Also note that join can take a generator expression, so no need to convert to a list first. I also chose more descriptive variable names here.

You should also give your functions (and classes) a docstring (by putting a triple-""" delimited string as the first line of the funciton body, as I did above), which you can access interactively via help(function_name) and which is used by many documentation building tools.

It also seems a bit too manual to have to know which method to call depending on the url. Your crawler could decide that on its own:

class Crawler(object):
    sites = {"hackthissite.org": ('.blight-td', 1, 0),
             "certifiedsecure.com": ('.level_progress_details', 0, 1)}

    def __init__(self, url):
        self.url = url
        self.options = self.get_sites_options(Crawler.sites)

    def get_sites_options(self, sites):
        for site, options in sites.items():
            if self.url in site:
                return options

    def get_site_data(self, user):
        """retrieve data from site and"""
        request = requests.get(self.url + user)
        return bs4.BeautifulSoup(request.text, 'lxml')

    def get_experience(self, user):
        select_str, index, out_index = self.options
        experience = self.get_site_data(user).select(select_str)[index].getText()
        return get_num(experience)[out_index]