Hoard of the Dragon Queen [IC. 2014 rules]

Dragon str save 26

While Draylin arms the ballista, Thistlewick and Montar both manage to land an attack on the fast-approaching dragon. For such a huge creature, the necrotic cantrip of the druid feels like the bite of a flea. Montar’s crossbow bolt feels like a toothpick between its scales, but at least it strikes deep enough. For a second, the blaze from the dwarf's fire rune envelops the dragon, but the beast moves in a spiral and shrugs off the chains with a mind-blowing fluidity, easily avoiding getting restrained.

Governor Nighthill and Escobert have moved toward another of the ballistas and spend their turns arming and aiming it.

The defenders of the third group have positioned themselves on the northernmost parapet. They too unleash a volley of arrows toward the beast.

Attack: 20 Damage: 5
Attack: 18 Damage: 9
Attack: 9 Damage: 10
Attack: 16 Damage: 7
Attack: 7 Damage: 9
Attack: 15 Damage: 6
Attack: 9 Damage: 6
Attack: 13 Damage: Unable to parse dice roll.
Attack: 8 Damage: 6
Attack: 6 Damage: 6

Over to Tam. Aiming the ballista and shooting it each require an action. 

[Can Tam use both of her actions, one to Aim and one to Fire? It seems like the Utilize action would still allow me the attack action. And the way you described it, one to load and one to aim and fire seems to support this. If this is wrong, let me know.]

Tam takes a deep breath and assesses the dragon, using her keen observational skills to line up a perfect shot. [using bonus action to activate Insightful Fighting. 12 Insight check versus the dragon's Deception to allow me to use Sneak Attack]

Aiming carefully, she unleashes a shot, not sure if it is good enough to hit [8 roll + 1 dex + 6 = 15 to hit the dragon with the Ballista. If I get proficiency bonus to that it is 17, but I doubt I do. In case that actually hits 22 damage + 9 for sneak attack in case that went through].