So I was soloing Karazhan yesterday and saw this amazing thing I looked over before I lost four times on the Chess Event. When the Echo of Medivh cheats he could put fire on the chess board. Now, when the enemy king decides to stand in the fire, he takes damage as well. This is one of the reasons which helped me to beat the Chess Event (not for the first time however) after some horrible bad luck.
Get in king, use heroism. Then I move into the peons in front of the king and the knight and move them 1 square ahead. I move into the knight and move him in front of the king and make him face forward. By this time Medivh should be close to cheating, which will probably be fire on the board, usually where your king is. So then I move into my king and move out of the fire. Also by now some of the enemy pieces have moved in range for my king to hit them. I use Heroism on cooldown and keep hitting everything. If you're lucky the enemy king will move in front of your knight and you will kill him easily. If you're unlucky then your knight will also be on fire and will die fast, which happened to me four times straight :'(. On the fifth attempt however I managed to stay alive for quite a while, but pretty much all pieces were dead except for my queen (the mage) and a bishop (the priest) The enemy had the king and two other pieces I can't remember. Their king decided to kill my queen and then proceeded to move into the fire so he could hit me. I was lucky enough to have more health than their king but nonetheless, this fire tip is golden, You should try to lure the enemy pieces into it. (Fire will usually spawn at king so use that to your advantage.)
PS. A reason to go for Karazhan's last boss is the head token for T4, which was pretty epic for Warlocks.