A basic tenet of screenwriting is that the protagonist must face down the antagonist near the end of your screenplay.
Though this seems obvious, your script should not make it obvious that your protagonist is going to catch up to the antagonist to “sort him/her out.” The antagonist should be seen as a powerful opposing force to your protagonist's plans. The antagonist should also be perhaps just a little more powerful or have better resources at his/her disposal, making the protagonist's job that much more difficult. We can see that the odds are stacked against the hero, and we're wondering how he/she will overcome them. Whatever seems the most obvious way to overcome them should be tried, but the protagonist should fail here. This sets up the moment when he/she believes all is lost and there is no way to defeat the antagonist.
The showdown that comes must evolve naturally from the story, and should not feel contrived. When you have decided how you want your story to end, you can place clues and foreshadowing earlier in the script in order to make the final showdown scene a natural evolution of the story. No “deus ex machina” that drops in to explain and save the day is permissible. The resolution of the conflict must come as a result of the protagonist's own actions.
The approach is fairly straightforward. When the protagonist realizes what must be done in order to defeat the antagonist, a realization that often includes the likelihood of his/her own death, the protagonist knocks off the antagonist's henchmen, one-by-one, from the weakest to the strongest until there is only the antagonist remaining. The understanding that his/her life is on the line is able to free the protagonist to do what must be done. He/she is no longer holding back in fear of death, he/she is pushing forward contemptuous of death.
It's a time-honored approach, and when executed properly feels organic and inevitable, which is exactly what you should be striving for.
Brian G. Walsh teaches screenwriting and provides script coverage for You Go Far Productions, Ltd., a film production company located in London, England. Brian is a two-time Screenwriting Fellowship Finalist, and his supernatural-thriller was a Finalist in the 2014 Creative Worlds Awards international screenplay competition. Brian has been a judge for three international screenwriting competitions, and is the author "No Place For Mercy," a short story Kindle book anthology available on Amazon.com.