In this specific example I don't think it needs nerfing, I mean in general, just because it's been nerfed/buffed once doesn't mean its balanced.
Create a second spell that removes the buff, and then triggers the original spell again. Put a trigger on the original pointing to this new spell with an amount="X" where X is the number of turns you want between resets. EDIT: Actually come to think of it, you'll need a second buff too, just for flagging purposes, so that the brittle buff can actually break and stay down. So two extra spells to do this.
