Well, for Toxapex in the story, I'd say a great moveset is Venoshock, in addition to Recover, Toxic Spikes, and Haze. Venoshock deals more damage when the target is poisoned, which pairs well with Toxic Spikes. Recover, of course, is there to keep Toxapex alive. Haze can be used to clear out any stat changes on the field, which can be really handy in certain story battles.
One effective moveset for Cofagrigus could be Shadow Ball, Will-O-Wisp, Protect, and Toxic. These moves give it a mix of offensive and defensive capabilities.
One possible fanfiction could be about Ash forming a special bond with Salamence. Maybe Salamence is a wild Pokemon that Ash manages to befriend through his kindness and understanding, which is different from his usual way of catching Pokemon. This could lead to an adventure where they face a powerful enemy together and Salamence's unique abilities help them win.
One interesting Salamence lemon fanfiction could be about its unique bond with a trainer in a romantic and somewhat spicy context. However, such fanfictions are not always easy to find as they are a very niche genre.
Salamence's appearance can add to the scariness in a horror story. With its large wings, sharp claws, and fierce - looking eyes, it can be a very menacing presence. When described in a spooky setting, such as a fog - covered forest or an old, haunted building, it creates a very creepy atmosphere that makes the story scarier.
There was a town near a mountain where Salamence were said to live. One day, a girl saw a Salamence flying overhead. It looked different, with a menacing aura. That night, she heard scratching at her window. When she looked out, she saw the same Salamence. Its claws were long and sharp. It seemed to be trying to get in. She was so scared she hid under her bed until morning.
Perhaps Salamence gets injured and Ash has to nurse it back to health. During this time, they form a strong bond. Later, Salamence helps Ash in a crucial battle against a powerful villain in the Pokemon world. This would show the power of friendship between Ash and Salamence.
Maybe it starts with Ash on his journey and he stumbles upon a Salamence in a unique situation, like it's injured or lost. And he decides to catch it to help it and also add it to his team.
You can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'Ash Catches Salamence' in the search bar and see what comes up.
Another option is to ask in Pokémon fan groups on social media. There are groups where fans discuss all kinds of Pokémon - related things, including fanfiction. Just be polite when asking and be prepared for the fact that not everyone may be into this particular type of fanfiction.