Specifically, in what part of the plot you need help? The cave part?
Here is some ideas:
To finally be able to beat the evil entity, the party must obtain powerfull item from their world. Now, there are several ways to make things interesting, and here is one way: The world where the deity transported the party and the boy is "blocked" by some unknown force. Perhaps the evil entity is trapped in the world where the party was transported, and it can't leave that world yet, though the magical barriers are weakening. So, what if the party must first find a way to deal with the barriers to be able to return to their world to optain the item, then find a way or negotiate with deities to be transported back to actually be able to deal with the evil?
Also, when they finally reach the evil entity, what about it making a counter offer for sparing it's life? Much higher reward would be interesting counter for the fact that the party would be betraying one of the gods, which would bring interesting depate inside the party.
Some ideas of mine, I return to the topic later with more time to actually think...
