Against Highlander Decks:

• The Darkness

Against Quest Mage:

• Loatheb
• Unseen Saboteur
• Dirty Rat

Against Secret Mage:

• Loatheb
• Eater of Secrets

Against Jade Druid:

• Loatheb
• Skulking Geist

Against Mechathun Warlock:

• Loatheb
• Unseen Saboteur
• Dirty Rat
• Skulking Geist

Generally good cards

• Coldlight Oracle: For milling your opponent + card draw
• Boompistol Bully: To stop battlecry cards
• MC Tech: For stealing your opponent's big board

And finally, for getting these nice effects again and again:

• Shudderwock
• Doppelgangster
• Grumble
• Zola