This encounter is all about positioning like the Crone fight. The minion next to the blue portal will only take one damage, while the minion next to the red portal has Windfury. Netherspite will try to retake both portals and then kill you with his spells and hero power. The easiest way to win is putting a taunt minion next to the blue portal as Netherspite will stop attacking at all and only use spells as long as that minion is alive.
Your first turns will usually be quite passive as you will most likely only use your hero power to put totems next to the blue portal. You should always block the blue beam first as it has no use to attack Netherspite will the blue shield is still active. However, if you only block the blue beam, minions with small life can die to Netherspite's windfury even though they only take 1 damage per attack.
So ideally you block both beam in the same turn, or you start with the blue one and a minion with high life like Sen'jin Shieldmasta.
As soon as you control both portals, start your attack against Netherspite's lifepoints. Wolfrider is a great choice for the first minion to block the red portal as he can immediately attack. Buff the minions under red portal influence with Flametongue Totem or Rockbiter Weapon to maximize your attacks.
Keep your (taunt) minion at the blue portal alive to protect you from Netherspite's attacks. Once you have control over both portals, this fight should be relatively simple.
Video Guide with play example (German):