At the title screen you have a number of options. Choose adventure. You'll start from level one. You'll start, specifically, as a fish named Andy the Angel fish. Complete his missions (levels) and you'll unlock a new fish to play the next levels. Andy has the first 8 levels of the game. Complete all 8 of them and you will have this achievement. If you die during this process and run out of continues (somehow...) you can return to the level you last completed and continue from there, with your score reset. The achievement will still unlock though after the completion of Andy's levels, despite how many tries it takes you.