If you win the coin flip, you can use any attack of a benched Pokemon. If the Flygon Lv.X is indeed on the bench, then you could use it's X-Treme Attack to do 150 damage to it, or any other Lv.X they have in play for that matter.
Unown G does not block the damage of E-Xtreme Attack, because doing damage is not an effect of an attack.