To hell with it Gun. Go full auto and resolve them all individually.
And here's how.
Just get together with yer GM before hand and "Pre-Roll" the attacks. Roll your your hit pool, Once, get all your hits. (you can Pre-spend your edge here if you want just remeber it won't be avalible
during the game.) Add up your hits and spread the hits between all the grenades.
This way you will still have some, potential, scatter to simulate RL but you can directly and qucikly affect all the grens. Write down the results on a notecard or something. Then when the time comes to fire your GM figures up the mods to the shot and to be fair every 2 pts of negative modifyer in that fight he removes 1 of your succeses form any of the grenades your rolled.
Now when the GM has to roll damage Have him roll everything that NPC possibly can. Then apply those results to every one of those grenades.
Ex. if he gets 4 hits on his reaction, or dodge or whatever. Then he gets 6 hits on his damage soak roll. Just use that result against every explosion. and remember, THIS would be the time for that NPC to use edge!!
Oh and for any of you out there about to post how it's OP or Unfair or, whatever else your going to whine about, to have the NPC just roll once for all that. Just think about how much damage that really is. You are DEAD either way. My way just speeds it up and moves the game forward. Oh and just on the flipside if some big bad Azzie goon squad hits you players with one of these, cause once you allow it into the game
IT'S THERE and that means for everyone,
I fully endorse useing this method on the PC's too. You can just speed thru to the Burning that Edge point to live and get on with the game