sorry,
in that case you have to do the following (took me a while, this puzzle is spicked with red herrings) :
Your turn
Put the Contested Cliffs, then the Goblin Pyromancer on top of your library with the Network.
Threaten the Groundshaker (leave open forest forest mountain), attack with it, Sledder, Slateback and the Biologist (12 dmg)
activate Sparksmith targeting the Groundshaker (2 dmg to you)
Cycle the Thicket, play the drawn Contested Cliffs.
Sacrifice Sparksmith via Sledder to make the Elven Riders 4/4, activate the Cliffs and let Groundshaker (with 2 dmg on it) + Riders take each other out.
Opponent's turn
He will play the Brackus.
Your 2nd turn
You play the Pyromancer you've drawn and attack with a 4/1 Sledder a 4/3 Slateback and a 1/2 Biologist.
He has two ways to block, but either way he will take 5 dmg (or 6 if you sac the Pyromancer
) and he'll die.