This was on the PS4.
When you use his ability Dervish, what happens is your first troop takes 5 damage, your second troop takes 5 damage. Then all the troops shift their position so your first troop is now last, second is now first, third is now second, and fourth is now third. However, the third troop being in the 2nd spot now takes no damage and the spell damages the now 3rd troop (previously the 4th). Then the effect ends (it doesn’t redamage the previously first troop even though it’s in the last spot now).
So the 3rd troop avoids any damage because it gets moved to the 2nd spot after the damage ball has already passed that location.
I can upload a video of it later when I get back home.