Well, the dice sort of decide if he is hit or not. You roll to throw the grenade, he rolls to defend against that and the result determines whether or not he was hit.
It sort of goes like this:
- state the intent of your action
- decide on an action type (attack, maneuver, etc.)
- roll the dice
- determine what happened from the dice roll
So your "hitting a joint" example would look a bit different.
If you state "I want to hit his arm joint", that's not so much an attack as it is a maneuver. An attack is simply a roll to put the hurt on someone, but that's very broad. A "called shot" like that would fit better as a maneuver.
Next, you roll. If you rolled a +7 total, and the guy rolled a +8 or higher, bad luck, you didn't hit him. You aimed for the joint, but he rolled to one side in the last moment, and the blow just went past him. It's not only your roll that determines a success, but the difference between the two rolls.
Though the same sort of goes for an attack. Only here, if he has armor, it might be that he rolled lower than +8, but he had enough armor to soak the access stress. Though that seems kind of overkill. Regardless, if the guy has high skills like that, it's going to be tough to beat him. The only way to do so is to gather up tags and use them all on one powerful attack.
I would suggest to talk to your GM. In my opinion, Fate plays better if you show your cards, so people can spend their actions on doing cool stuff instead of wasting them on things that won't work. If the guy really is flawless, it seems like a boring opponent, as well.
