So from my understanding of the ability dampening based on wowhead information on it the weak aura would actually be very simple to make. I haven't tested this as I am not a PVPer and wouldn't make it to the point where the debuff gets placed on you, but it should still work with no problems.
Code:
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This Weak Aura has no custom coding in it and will explain what it does so you can learn as well. (Should this not work to your liking please feel free to reply here and I can make modifications as you need, within reason.)
Creation
New > Progress Bar (As that is the type you had in your sample provided)
Trigger
Type: Aura
Aura(s): Dampening
Unit: Player
Aura Type: Debuff
Own Only: Checked (This only looks if you have the dampening debuff)
Load
Instance Type: Arena (This means that the weak aura will only run while in an arena. So you can be in your garrison and it is still not looking for the dampening debuff)
Display
Configure the bar to your liking
If you want to be able to see the stacks you can replace the right text (%p) with "%s" and you can see the stacks that you have of the debuff.
If you have any other questions feel free to let me know.