Just remembered this fic. I think I've asked about it here before, but I don't remember.
Firstly, I remember that the author's name had something to do with mushrooms, I think.
Secondly, the story was a monster invasion story. Ranma, Ryoga, Kuno and a few others are involved in repulsing the monster invasion. Ranma has an OC girlfriend named Sunny, I think her real name was Sun Yao. Ranma uses a .50cal machine gun in at least one scene. Ryoga has a titanium locking bracelet with built-in GPS. Ranma gets put on trial for many things including treason, which is dropped when it's discovered that the plaintiff lied about Ranma's age.
And I think it begins with Ranma being sold into some kind of interdimensional gladiator/slave system, where he spends several months before being released.
I'm not sure how many people here would actually like this fic, but I did at least a bit, and I'd like to find it again, if anyone recognizes it.