You could try searching on popular fanfic websites like Archive of Our Own (AO3) or FanFiction.net. Just type in 'harry fights the zerg' in their search bars and see if any relevant stories come up.
The 'harry fights the zerg fanfic' sounds like a very creative concept. It could explore how two very different fictional worlds collide. Harry might have to adapt his way of fighting in a completely different environment. The Zerg are known for their large numbers and aggressive nature, so Harry would need to be very strategic. Maybe he teams up with some characters from the Zerg's own universe or uses some kind of magic to turn the Zerg against each other.
One common theme is the revelation of secrets. Often, Harry discovers something about Snape that makes him want to fight, like Snape's connection to the Dark Arts or some hidden actions he has taken against Harry or his friends.
Perhaps in this fanfic, Harry teams up with some of his Hogwarts friends. They combine their magical knowledge and courage. For example, Hermione could use her research skills to find a weakness of the Daleks, while Ron provides support and loyalty in the fight. And Harry, being the main hero, directly engages with the Daleks using his powerful spells like Expelliarmus but modified to have an effect on the Daleks' technology.
Maybe Harry discovers a hidden part of the wizarding world that uses guns as a form of magic - enhanced weaponry. So he has to learn how to use these guns in combination with his magic to defeat a powerful villain. For example, the guns could be enchanted to fire spells instead of normal bullets, and Harry has to master this new form of combat.
In a fanfic where Harry fights the Chitauri, he might use his magic in unexpected ways. For example, he could use spells like 'Expelliarmus' to disarm the Chitauri weapons. His ability to apparate could also give him an advantage in dodging their attacks. Maybe he would team up with some of the Avengers as well, sharing his magical knowledge with them.
Perhaps in this 'harry fights back fanfic', Harry discovers some new powers or spells that he uses to fight back. He could be tired of being pushed around by Voldemort's followers or other bullies in the magical community, and he uses these newfound abilities to protect himself and his friends. For example, he might learn an ancient spell from a hidden book in the restricted section of the library. This spell gives him an edge in combat and he starts to turn the tables on his adversaries.
Well, it could be that Harry Potter discovers an evil empire within the wizarding world that has been secretly controlling things. He then uses his magic skills and the help of his friends to fight against it. Maybe there are new dark spells and powerful artifacts involved in this fight.