Could use standard counterspell rules. Adjust the cost so it can be done defensively. Have the counterspell effect be based on might + weapon instead.
To make it more palatable to players put it on an IoP?
I don't understand this post at all. I would like to use standard counterspell rules, but the issue with using them would be calculating stress if the defensive action fails. If I'm rolling against power (and not the targeting control roll) and fail, then what? If I'm not rolling, I don't like it. I would know exactly how strong of a spell I can stop, with no chance of risk.
This is something I ran into when a player wanted a stunt that allowed a counterspell to be done as a defensive action. On a successful counterspell, there are no problems. But if you fail? Do you then also get to roll a defense roll? I don't think this should give characters a second chance to succeed.
Since the issue stemmed from counterspells working against power, while defense rolls worked against targeting (read: control), I simply changed it so that it in the case of evocations it would work against the control (which would likely be at least as high as the power in the majority of situations, if not higher, weakening the power a small but acceptable amount).
I don't understand what you mean by adjusting the cost. I think -2 is pretty fair. It's not as good as full evocation or other -3 options, but is certainly better than every -1 option in the game. How do you think the power should be costed?
In my original post, I said that it was going to be part of an IoP (in this case, an axe). But I didn't want a critique of the entire axe, just this bit.