If the character were to claim that he was attempting to punch the grenade into his opponent and then pull back before it detonated then you could perhaps use a fists roll +4 damage for his opponent and attack roll zero attacks against the rest of the zone with weapon 4 as he isn't actively targeting them, including himself, though applying ambush rules to the player in question as they would be short on time to protect their soft bits (face in particular). Even if using endurance for defense rolls through a stunt I think it should probably be subject to ambush rolls in this situation just like anyone else who's flat-footed against an attacker (though in this case the attacker would be themselves) otherwise it represents a form of immunity to ambush rules and should probably be represented as a higher value power instead.

If the character is making no attempt to pull back before the grenade detonates then perhaps their fist roll should count against them as well to represent their ability to close the distance with their target rather than fumbling the grenade or being forced to toss it at the opponent instead, though this would be foolish enough that I'd hope it was attributed to an aspect so that I could give a Fate point for it, maybe two if they injure themselves badly enough.

Of course handling it as a concession to end the fight could work as well, though to take out a miniboss/lieutenant in the same blow they'd have to negotiate a steep consequence if they weren't already winning the fight.