I think the cost is fine. You need a higher attack, because once you play it, you need it to kill the other player. You can't expect them to discard 8 15 GotP before they beat you with the 8 cards they already played. They will have to be an atacker along with their other things.
Thing is, with that high of a cost, I'm afraid that GotD will become the Time splash in Darkness/Aether decks rather than Nightmare to Time decks as Fractal + Pest + Nightmare + GotD has amazing potential to completely lock down your opponent and actually force them to either throw away their essential cards or GotD. Like the other Time creatures, probably excluding Dragon, GotD has prefect synergy with another element, but the difference is that Pharaoh, Anubis, Vuvu, and even Fate Egg (though to a lesser extent) can be viable under a Time deck with a splash (
,
,
/
,
). This is a buff to Darkness than it is for Time for that reason.