Defence is the best offence question

Ok so I am looking at all of the levels on pure strrategy and they say "best wave: 11" or higher. So my question is why do I not have the achievement?

Start a new level and lose

just play up to wave 10 and sell all the defense and let the monster destroy your core... :)

Not necessary just start any level on any difficulty and lose.

well i went into the first level on medium again and only put down two electric auras because just losing is not in my blood. I though I would go down in the first few waves but I got to wave 11 before they finally got though (only one did lol) and I lost and got the achievement. So, I was happy about getting the achievement and very surprised at how well only two electric auras did.