Look at the fields
Data - Damage Interval
Data - Damage Amount
Data - Max Damage
for the ability in the object editor.
if
[Damage Amount * number of targets in the AoE] is less than or equal to Max Damage
or
Max Damage is set to 0
then:
Damage dealt to each target in the AoE will be: Damage Amount points of damage.
else:
Damage dealt to each target in the AoE will be: [Max Damage / number of targets in the AoE] points of damage.
Shortly: set Max Damage to 0. Then every Damage Interval seconds damage dealt to each target in the AoE will be: Damage Amount points of damage.
Do you understand how the fields Damage Interval and Effect Duration make up the total number of waves of damage?