First of all, Gandalf can't explode efreet, but he is slightly better and mutually assured destruction. So Gandalf is not necessarily better than Shi. Even if he is stronger, he is not 100% sure of success and walks away. Gandalf has no reason to risk his life.

As far as figure is concerned, Stone is much bigger than Everett. In addition, the ancestor dragon has a high intelligence after being created, and even can use some simple spells, so the stone has an advantage in the battle of wits.

In Middle-earth, it happens from time to time that human beings born in flesh slay dragons with courage and wisdom, but the purpose of Gandalf's formation of the Gushan team is not only to get rid of the dragons entrenched there. He wants the Thorin oak shield, the blood of Turin family among the dwarves, to regain the glory of the dwarves and reoccupy the once glorious Irubo. In this way, the dwarves in the north can reunite and prepare for Sauron's return in the future.