Hi, this is the scenario: I normal summon hand of nephtys. I activate Last will, I bring Phoenix by it´s special effect and then because of last will I call Sangan. Then when I summon this last monster I chain Torrential tribute. Set a card face down, end of my turn.

My opponent attacks and enconuters my sakuretsu armor. He sets a face down trap card (bottomless trap hole)

then my turn comes: I draw a card, which results being Mystical Space Typhoon, and since it´s standby phase, I activate it from my hand to save the Reborning phoenix.

My opponent says I can´t do that.

I say it´s a quickplay magic card, and thus when I draw it, I have the priority to decide which effect activate first (mystical, then phoenix´s) so, please bear with me, and could someone help me to resolve this scenario?

Yes...this is completely legal to do...this is why it worked with Mirage of Nightmare...

You know, you could just say you're activating Mystical in the draw phase, after you draw and before you enter your standby phase so there's no confusion. But yes, what you did IS perfectly legal. Also, do not use the word "priority" here. Priority has nothing to do with this scenario. As the turn player, you get to choose the order you activate effects that can be activated in the Standby Phase, but it's not a "Priority" matter.

By the way, friendly advice: Next time, activate Last Will BEFORE you summon Hand of Nephthys. If you summon Hand and then activate Last Will, they can chain to Last Will with an effect that would prevent you from activating Hand's effect at all and then you wasted a Last Will. If you activate Last Will first, THEN summon Hand of Nephthys, you retain priority after the summoning to activate Hand's effect and there's almost nothing they can do to stop you.