Already been mentioned, but Spirit Fox. I simply refuse to enter a PvP battle against it. Much rather refresh and find a new battle not designed to annoyed me and prolong the battle.
Traits - Stealthy and Empowered. No thank you.
Spell - True damage, mana drain and board control. Stop, it hurts.
And to top it off it has a low mana cost which it usual gets back after it casts. Ready to drain you again and repeat the process.
Bone Dragon has nothing on that vermin.