Ballista is one of the forms of Conflict (pvp) in Final Fantasy XI. With the recent wave of patriotism sweeping Vana'diel, the decision was made to reintroduce Conflict as a form of friendly rivalry between the three nations. Ballista is based upon San d'Orian military training exercises that involve participants digging up small stones called Petras to toss into goals called Rooks scattered around the battlefield.
You must have earned the title "Gold Ballistar" and possess the key item Gold Ballista Chevron. The Gold Ballista Chevron will disappear from your inventory after about a week, nullifying your ability to earn this achievement, so be sure to unlock the achievement asap. Refer to this thread for details on earning this achievement. For more information on Ballista, click here.