That is generally why I prefer to play by tournament rules. If neither player can kill the opposite get mage in 75 minutes, the game ends in a tie. It discourages turtling beyond the first few turns of preparation, though I admit that most of the books that I play against are capable of dealing major damage by the beginning of turn 3, so turtle books are no real problem. If the other book turtles, I will just bring out a few creatures and start stripping away their defenses one by one, until it is just their mage, alone and afraid, against some very nasty creatures.