When her attack hits home, she takes aim at a new target. Before draw her arrow, she makes a quick, magical gesture towards it, and then lines up her shot. This arrow glows with a sickening, yellow energy.
Also, for clarification's sake, I used Colossus Slayer on the first attack.
Rolls
Attack 1 vs Duergar 12 - (1d20+11, 1d20+11, 2d8+6)
1d20+11 : (20) + 11 = 31
1d20+11 : (17) + 11 = 28
2d8+6 : (57) + 6 = 18
Crit damage - (2d8)
(88) = 16
Mind-Scrambling Arrow (if hit) vs. Duergar 10 w/ Sharpshooter feat and Hunter's Mark - (1d20+6, 1d20+6, 1d8+16, 3d6)
1d20+6 : (20) + 6 = 26
1d20+6 : (3) + 6 = 9
1d8+16 : (1) + 16 = 17
3d6 : (526) = 13
Another crit? - (1d8, 3d6)
1d8 : (5) = 5
3d6 : (346) = 13