The ballista crew chief uses material from the book Heroes of Battle

Ballista Crew Chief    CR 1/2

Male Human Warrior 1

N Medium Humanoid (Human)

Init +1, Senses Listen +0; Spot +0

Languages Common

AC 14, touch 11, flat-footed 13 (+1 Dex, +3 armor)

hp 5 (1d8+1 HD)

Fort +3, Ref +1, Will +0

Spd 30 ft.

Melee short sword +1 (1d6/19-20)

Ranged Light ballista +2 (3d8/19-20) or light crossbow +2 (1d8/19-20)

Base Atk +1; Grp +1

Combat Gear light ballista, 20 ballista bolts, light crossbow, 10 crossbow bolts, dagger, studded leather armour

Abilities Str 11 (+0), Dex 13 (+1), Con 12 (+1), Int 9 (-1), Wis 10 (+0), Cha 8 (-1)

Feats Ballista Proficiency, Weapon Focus (light ballista)

Skills Climb +3, Profession: (Siege Engineer) +2

Combat Edit

Ballista crew chiefs are supported by a team of 4 first-level commoners, each with 4 ranks in profession: siege engineer. Each commoner that succeeds on a DC 10 aid another check adds a cumulative +2 on the crew chief's attack roll when they fire; with four commoner supporters, two use the aid another action each round.

Loading a new bolt in a light ballista requires two full-round actions, which means two crew members can load the ballista in a single round. This allows the crew to get off one shot per round.

This build allows the use of ship-mounted ballistas. If the ballista is used in a battlefield situation, consider the possibility of heavier armor.