So a good Grand Challenge deck, taking into consideration the King Level reduction/max level for cards-
Miner
Furnace
Mega Minion
Poison
Witch
Hog
Valkyrie
Elixir
The general strategy of this deck is overwhelming the opponent. Once 60 seconds hit, having 2 furnaces out is good for overwhelming a tower, for example. At level 7 for example, the level 9 fire spirits take 2 arrows to take down, meaning it takes 2 arrows to take one down, meaning even if a furnace is alone, always one spirit will hit an opposing tower.
Miner I keep for attacking high value targets. Being Princess or enemy Elixir. If it's the enemy Elixir, I like to cover it in poison as well-If they send something out to try and take out my miner, atleast it'll be killed(If it's something like skele or minions), or damaged and slowed. It's basically forcing the opponent to go 'I can counter his miner, but whatever I send out will be hurt, it might not be worth it.)
Valkyrie and Witch I keep around for countering pushes. Which for Giants, as the skeletons will add up in damage. Valkyrie for Hogs and packs of enemies(Goblins or Barbarians). If you have a furnace active, it'll help in anti-air and anti-pushers, as well as beef up that lane in general. Witch is also the go to for countering minions, or in worst case scenario, dropping a Poison.
Mega Minion serves as a Mini Pekka of the air. Can counter Giants or hogs. Can't be countered by zap, arrows, Princess's, or Fire Spirits. Can't be overwhelmed by ground units. He's a very, very strong, versatile card. His push is pretty great as well, the damage he does adds up. He rounds up my deck-I never using him to push directly, just keep him in hand to counter a push.
Hog is there to do a heavy push-Usually on the lane w/ the Furnace, or being covered by Poison. If for example, they send a Giant to your lane, and you counter it with a witch/mega minion, by the time they're dead, you can bust out a Hog who will be bolstered by Witch/Mega Minion. If you have the mana(Through elixir output or stocked up), you can cover their path with Poison.
It's a decent deck. Always gets me atleast 3 wins in Grand Challenge. usually overwhelms the opponent unless you fuck up yourself.