Tempo Mage has seen a boost in popularity since some of the new Karazhan cards came out. MrYagut is one of my favorite streamers, and this is the MrYagut Tempo Mage Karazhan w Barnes he was playing on stream.
Barnes is as good as most people thought it would be. It’s not as effective in Mage as a control deck with lots of Deathrattle minions, but minions like Bloodmage Thalnos, Cult Sorcerer, Flamewaker, Azure Drake, Archmage Antonidas, and Ragnaros all have passive effects that can help swing the tempo of the game.
Try to drop Barnes later in the game when you have some spells to back him up. Don’t forget too m Barnes can only copy a minion that is still in your deck, so remember what you have and have not played.
General Tempo Mage Mulligan Strategy
I like to have Mirror Image in my opening hand, along with any of the 2 drop minions. I’m not a fan of Arcane Missiles as you’re just going to hit face trying to snipe a single early game minion.