Why couldn't Gandalf kill multiple orcs at a time?

Question

We see Gandalf fight and defeat the Balrog. He loses his own life in the process, but through this battle we learn that at the very least he is as at least as powerful, if not more powerful, than the Balrog.

Fast forward to The Return of the King and the armies of Sauron are attacking Gondor. We see hordes of orcs rushing in off of their siege towers and Gandalf fighting them off with his sword and staff, but doing so one at a time. I can't help but think of the Balrog at this point... If the Balrog were entering Gondor, or defending it for that matter (although that would never happen) it would be smiting everything in its path dozens at a time. Although we never actually see the Balrog fighting anything save for Gandalf I think it is pretty safe to assume it would be incredibly formidable against whatever were in front of it, even a dozen trolls or more.

So if Gandalf is more powerful than the Balrog, why couldn't he do the same to the orcs as he did to the Balrog? Why is he fighting one orc at a time with his sword and staff instead of decimating dozens at a time with some form of magic, be it his ring, his staff or any other form of power he may possess?

As far as crowd control goes, Legolas, Gimli and Aragorn are actually shown as much more of a threat to Sauron's army than Gandalf is. All Gandalf really ever did was shine a flashlight a couple times, other than that it was just one-at-a-time swordfighting.

Answer 1

Under the right circumstances, Gandalf could kill multiple orcs at once; he's done it in the past:

But not Gandalf. Bilbo's yell had done that much good. It had wakened him up wide in a splintered second, and when goblins came to grab him, there was a terrific flash like lightning in the cave, a smell like gunpowder, and several of them fell dead.

The Hobbit ch. IV: Over Hill and Under Hill

However, Gandalf's essential task as a wizard (one of the Istari) is to inspire and advise the Free Peoples in fighting Sauron, not to do the primary fighting himself:

But they [the Istari] were forbidden to match his [Sauron's] power with power

LOTR, Appendix B

Also, there's a difference between killing a small group of orcs and a large organized army.

Gandalf's ability to defeat a Balrog at the cost of his own life doesn't mean that he would be as effective against a horde of orcs as a Balrog would be.

As one of the Istari, Gandalf's body is fundamentally human in nature (though either ageless or incredibly long-lived). Saruman was killed by an ordinary Man (Grima) with a knife. Gandalf's status as a Maia wouldn't necessarily protect him from being swarmed and killed by a large group of orcs.