Target will be chosen randomly if it's a Hero Power like Fireblast.
And of course the opponent will pay the mana required to use his Hero Power, that's the point of the card.
Edit: The mana cost is back to 0 now but changed the condition for activation, now it will activate after your opponent plays a card and not at the start of their turn to give a chance for your opponent to play aroud the secret.
The secret won't activate if your opponent used their Hero Power before playing any card (unless they have a card like Coldarra Drake on the field) or if they don't have enough mana to use their Hero Power which make the combo with Saboteur less consistant.
3
The Arcane spell: Arcane Explosion, Arcane Missiles and Arcane Blast.
4
http://wowwiki.wikia.com/wiki/Thunderhead_Patriarch
2
8
New here, this is my first card. The ice spell here: Frostbolt, Blizzard, Ice Lance, Cone of Cold or maybe add in Ice Barrier and Ice Block as well.
Edit: forgot Frost Nova.