All you need to do is sling, sling, sling...

Once you get to Baldur's Gate, your back-line, sling-wielders will get an option with slings. The sling +1, which is +1 THAC0 and +1 damage, and the sling of unerring accuracy, which is +2 THAC0, with no damage adjustment. Later in the game, another better sling may be available, but at this point in the game, which would you prefer? A bit more accuracy? Or a bit more damage when you do hit?