This can be done fairly early on in the game or pretty much any time at all. All you need to do is open up your map using the touch pad, find an artifact for currency, and then open up the Options menu to navigate to Weapons and Runes. Once there, go right to the bow, spend the currency to unlock the rune slot, and there's your trophy.
There are obviously other ways to get the currency, but this allowed me to get into 5 minutes into the free play portion of the game.