Circle of Flame is a sideboard card. A good sideboard card depending on the meta, but still dead against a large part of the decks. I'd probably max out on Incinerate (kills Cudgel Troll) and consider adding in Fiery Hellhound, since you're playing with Goblin Tunneler anyway. If you want to stretch the Tunnelers, you could max THEM out and play Immolating Souleater (New Phyrexia) and Fling, fit in a nice little combo?