You could keep track with stock loss. Like I said, if stock falls under a certain level then the Apocalypse can begin.
But the card doesn't define that it has to come with Mage tokens. As DPsycho said, one could take it in and leave the Mage tokens out of the deck, thus keep playing this card forever without fear of a penalty. Even with the Mage card, one could play it 6 times, then play another card and then this one to reset the counter and thus keep playing it all the time.
An alternate suggestion is to make it rely on a certain type of resource, like gems for example: Replace Mage token with Gems
Therefore the card would be:
Cost = 1 gem
If last card was not this card then Gems = 7
Always:
Enemy stock -3
Production x 0
If Gems = 0
Then both Towers -150
You will have the same result with a game Variable that is always present at every game, regardless of deck build-up.
I still feel that for a low-cost card though, the If Mage tokens/Gems = 0 effect is gonna be too much of a punishment for the opponent, since not only he most likely won't be able to play the card he wants to finish you, in the end he will lose as well. I suggest the effect to only apply to the player of the card.
Furthermore, since the player's stock would be too low if you implemented my suggestion, I'd suggest that you remove the Durable keyword and allow the card to replace a random card in hand, so as to give the opportunity to the player to win the game if he's lucky.
Therefore, the player would have 6 chances (maybe less) to bring a good card in his hand to save himself and stop the apocalypse, while he hopes that the opponent won't bring something better to finish him off. At the same time, the player won't be able to play something costing Gems, since they'd be too low and the chances lie with the opponent to beat him. In other words, this would be a last measure card with a glimpse of hope, as you intended I think, and it'd be a situable effect for a low-cost card.