My GM has created a Mary-Sue-type NPC. She is a demigod with thousands of years of experience, and was once an enemy, but is now an ally of the party, though really of only a few favourite players. She is literally undefeatable, and I actually mean literally, not figuratively. Any attempts to defeat her will be vetoed.
To compound the problem, she is also pretty much a jerk, though to the GM's credit he has actually given some attempt to give her a heart of gold. The invulnerability and jerkishness don't combine well, e.g. a nother player character tried to become friends with her and make up for previous grudges—denied and his head is being used as a football.
This situation will only be worsened by the problem that our next game will be in the same setting, except that our new characters will be nerfed, due to the fact that our whole group agrees our last characters were too OP.
The Mary Sue, of course, will stay on the same power level.
This is making the game quite a bit less fun for me, though I don't know about the other players. It is quite seriously ruining it, due to the fact that she has an extremely short fuse and will kill anyone who annoys her.
What can I do to persuade the GM to make her less powerful/obnoxious? Please help!