Shit. I always thought Mensa was tough to get into.
Like you were supposed to qualify as a genius or something. 
1250 is nothing.
All it requires is that your IQ be in the top 2%.
"All"? 98% of the Earth's population or >6 billion people will never reach that level.
Fortunately, anyone with above average intelligence would be able to get in with enough practise.
If by "above average intelligence" you mean a person with an IQ of above 100, then I don't think any amount of practice could change a person with an IQ of 110 into a person with an IQ of 132, which is the Mensa cutoff score on the Stanford Binet or Woodcock-Johnson IQ tests.