Meggido shakes his head emphatically in answer to Halgrim's question. He tries to visualize where the great siege engine had been, and shoot where he thinks someone using it would stand.

Spoiler
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Firing to hit the square where a person using the ballista would stand, giving total concealment to anyone in that square:

Attack: (1d20+5)[20]
Damage: (1d8)[8]
50% Miss Chance from concealment (1 = Miss) (1d2)[1]