Demoralizing Strike is a 1st level Ardent at-will. Of interest here is the Augment 2 (sample build):
Close: burst 1
Target: Each creature in burst
Hit: 1[W] + Charisma modifier damage, and the target takes a penalty to all defenses equal to 1 + your Constitution modifier until the end of your next turn.
By epic level, all things being equal, this turns into a -6 to -8 defenses debuff until end of next turn that can be repeated 6 times during the encounter, turning all monsters around the ardent into things that can be automatically hit.
What is the best way to determine what the debuff should be, such that the ardent can have a meaningful effect on battles (more than -2, which is just token) without trivializing them?