VS. ground Thors do 30 Damage per attack with 2 attacks. Hydras have 80 health. The Thor's attacks are staggered (not like the phoenix that makes both laser attacks at the same time).
Back to the question at hand: When a Thor is fighting 2 hydras (no upgrades on anything) will the Thor make a total of 3 or 4 full volleys? (1 volley being both attacks) After the first volley the target hydra has 20 health. So, does the next volley finish off the target hydra and hit the second hydra once or does the attacking delay trigger on switching targets ultimately requiring the Thor to make a fourth attack.