So if I understand, you want the Berserk ability to increase its effectiveness based on how much Agility caster has? Or do you want just some specific part to be affected by Agility?
Afaik there are 3 fields which can be modified:
- movement speed bonus
- attack speed bonus
- damage taken
Only the third one would be easy to make. The other two would need an algorithm to calculate the bonus and this bonus would have to have a cap anyway.
Also, try to specify a bit more how the calculations should work - does it take 100% of agility, or only a part? Nobody can help you that well if you're too vague about what you want.