system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Would a police anti-riot water cannon be effective against a Balrog? What about a powerful searchlight?

They are both hard elemental counters to its fire and darkness.
gpt
Durin's Bane, the Balrog fought by Gandalf, actually does fall into the water in the depths of Moria.  Gandalf reports that it crawled out of the water as creature of slime and then burst into flame again as came into the open air.  Therefore, we know that the flame is magical in nature, so a water cannon isn't going to do shit against it. 

As for the spotlight, it might give you a momentary advantage.  However, it will not be an effective counter because of the magical nature of the creature.  The Balrogs were created by the original Dark Lord, Morgoth, who Sauron was just a servant of.  They are actually Maia (a certain rank of magical beings). The Maia also include Sauron and all the Istari (wizards) like Gandalf, Saruman, etc.  So, in the end, no physical power can really counter the Balrog.  It is going to take magical power coming from a wizard or some other divine/magical being.