By the end of the story, we’d reached level 25, so it’s possible that you might have to grind this out during the endgame. Levelling slows down considerably after level 20, so completing every non-Story Quest will help a lot in getting to level 30. Just keep killing zombies and completing quests, and you’ll eventually reach the level cap and earn this achievement. (Note: This seems a mite glitchy, and got stuck at 96% in my achievement tracker).