If you can't afford the food or gold required, try to get the items you can earn or collect to get the gem cost as low as possible. For example, into the dark costs 31 gems when all skulls and keys have been aquired.
Logging out and logging in will spawn three more keys every time.
Visiting a friend will spawn three more keys every time. This is faster than logging in and out.
Horns are acquired through breeding. When an egg is placed in the hatchery, you receive one horn for every minute of it's standard breed time. This is not reduced if you speed up the breeding with gems. (Although if you're going to spend gems, you might as well buy using gems -- it's cheaper and faster.)
Shields are gained for each won PVP battle.
See Curse of Dramses for other tips on completing this event.