This trophy is super easy, as a Birdie is only one under par. Official round simply means you're playing Challenge Mode and there aren't any modifiers active (such as no wind, real golf, etc).
The easiest way to get this is to play the first challenge. This challenge should be super easy since there's barely any wind, the courses are pretty straightforward and the greens are pretty flat. More likely than not, you'll get this without even trying.