I've been giving this some thought already, since it has come up in one of Shennaqua's character concepts. Here is my understanding, with twice my normal caveats about how this could all be wrong...
I believe the Mastery power isn't intended to be a combat power.
** EDIT: I misread your example - see my next post. My incorrect reading, where I thought you wanted to weaken the punch, is what I talk about below... Sorry! **
Your example of using gravity to make an enemy's punch more difficult is something that could be represented already by other existing powers, like Weaken Agility due to the high gravity, or a ranged Deflect usable on others.
So letting a blanket power like Gravity Mastery effectively do the job of those existing powers (plus anything else we can come up with) triggers warning bells for me as a GM. At most, I might treat it as a limited form of Omnipotence, so using Mastery 5 that way might gives the character the equivalent of 1 rank of any appropriate combat power (or add an appropriate enhancement, etc.) Using an alternate example, like Omniscience, where there is a more limited selection of powers that can be emulated, maybe the max rank would be 2 for 5 points of Mastery. Certainly no more than that, and even then I'm a little dubious about allowing it.
I believe the intent of the Mastery power is primarily to provide non-combat environmental / situational benefits that don't really fit any of the "normal" powers. It lets you make some loose (comic-book style) justifications to achieve some goal and affect the environment around you.
Some examples for gravity mastery might include:
An empty building is on fire; use anti-gravity to make the air weigh less and flow upwards. Either it evacuates the oxygen, starving the fire and putting it out, or it causes surrounding air to rush in and possibly blowing out the fire. Extinguishing the fire is something that might take a group of firefighters several hours, so according to the Mastery chart that would be maybe a difficulty 15? A 10 would be something one person could do in a few hours, 20 is something it would takes weeks for, so 15 sounds about right.
A large occupied building is damaged in a super brawl and is going to collapse. Use gravity mastery to reduce the load on the weakened building so it doesn't collapse until you are able to rescue all the people! Reinforcing a big building is a superhuman feat, so maybe difficulty 30 from the chart would be appropriate? As a nifty side effect, anyone falling from upper stories might land without serious damage in the low-G field!
The villain has been defeated, but his underground lair is filled with dangerous devices that must not be allowed to fall into the wrong hands. A gravity master could cause the entire complex to collapse on itself with a difficulty 30 check (which can affect up to 1 mile area, like months of work with heavy equipment). I can picture it in a comic book now - one majestic page, or more likely a two-page spread, looks over his shoulder as he dramatically gestures and the vista before him implodes in a cloud of dust!).
And that is kind of how I plan to make my judgment calls about the Mastery power. We've all seen the comic books where the characters with Mastery (like Magneto, Storm, Thor, etc.) stop doing small-scale tactical things (like combat powers) and instead get shown in a two-page spread doing something really dramatic to their environment, like calming or creating a hurricane, or moving the entire Golden Gate Bridge (much larger than even Telekinesis 5 could support), etc.
In my opinion, the change to the environment might be dramatic, but it should generally provide strategic benefits rather than tactical one-on-one combat benefits. And if it does affect one-on-one combat, it generally should affect everyone in the area, so maybe creating a hurricane would cause the same game effect as the Confusion power, but it applies to both heroes and villains and any poor civilians within a few miles. That means it should only be used that way if there is a strategic goal that requires it and outweighs the cost to the heroes and the general populace.
And more than most powers, I think there needs to be some restraint on the player's part. Just because you have enough Gravity Mastery to make the villain's headquarters implode doesn't mean you should do it on page 3 of the issue! On the other hand, it does become dramatically appropriate on page 26 or so, after you've accomplished your main goal and want to have a dramatic final statement. Hopefully most heroes will be too concerned about possible loss of life to do it on page 3, but even anti-heroes hopefully won't do it off-the-bat either. Depending on your players you might need to have a plot reason why they shouldn't do it right away, but in many cases the player will avoid such anti-climatic choices on their own.
Does that help?
