I learned about this achievement from someone with the achievement. You can start on wave 19 and beat 25 to get the achievement. I tried, but it is crazy hard. I got to level 23 once in my 3 attempts. You only have like 3 lives to do it and I can't.
I did get the 2,500 kills while attempting. So, I think I've got all the achievements for this game I'm going to get.