A Wight King with Sword of Kings and Gem of Blood can work pretty well when in a unit with a Banner of Barrows, too. The King is likely to survive the first round of combat (the Gem of Blood can deflect one Killing Blow wound back onto the Assassin), and then take the Assassin down. The Assassin's only chance of survival is to kill everything first, and if you survive the initial barrage you'll be in a good position.
The Wight King will also not weigh you down needlessly, as it's a strong character killer with its 5+ Killing Blow, and is a durable Hero for you to give your Battle Standard to.