I heard the thunder clackers were pretty bad so I took a look at them myself with the following conditions:
Stats = 400 all
Light Armour = 200
Crafted leather armour at 295q/300q
Platinum steel Longsword at 285q
I rode by them repeatedly on my rivnak to get the debuff then attacked one of them with a weak magic spell to get it to attack me. I heard they were bad, but even I was surprised when I received two hits from the clacker both 1300+.

I beleive, like bonifarza, that I was not in any stance at that time. I know you want it to decrease deffence....but with that much damage being inflicted something is NOT working as planned.

...as it is now, thunder clackers are worse to fight than maulbernauts.
Just another thought, if this is a deffensive move on the clacker's part, it could be adjusted to only reduce the attack power. Reducing deff. along with attack kind of sounds like an aggressive move...but thats just my view on it.