Well, Sylveon's ability to calm others with its presence could be used in a story where Pikachu gets overly excited or scared. For example, if they are in a spooky old Pokemon mansion full of Ghost - type Pokemon. Pikachu might be too afraid to use its electric attacks effectively. Sylveon could calm it down, and then Pikachu could use its attacks more accurately. Also, Pikachu's speed with Quick Attack can be used to get Sylveon out of dangerous situations quickly, like when they are being chased by a large group of wild Pokemon. Sylveon can then use its ribbons to either hold off the pursuers or find a hiding place for them both.
In a battle against a powerful Rock - type Pokemon, their abilities can work together in an interesting way. Sylveon's Pixilate - enhanced Hyper Voice can chip away at the Rock - type's defenses. But the Rock - type might be resistant to Fairy - type moves. Here, Pikachu can use Thunderbolt to stun the Rock - type momentarily, making it more vulnerable to Sylveon's attacks. And when the Rock - type tries to counter - attack with a ground - based move, Pikachu can use its speed to quickly move Sylveon out of harm's way. This way, their different abilities complement each other and create an exciting battle scenario in the fanfiction.
In a story, their abilities can complement each other well. Let's say they are exploring a cave full of water - type Pokémon. Pikachu's electricity can be dangerous in water, but Snivy can use its grass - type moves to create floating platforms for Pikachu to stand on. Then Pikachu can use its electric attacks from a safe distance. Also, Snivy's ability to blend in with plants can be used for stealth, while Pikachu can act as a decoy with its bright yellow color and cute appearance, luring enemies into a trap where Snivy can ambush them.
One possible storyline could be Sylveon and Pikachu teaming up to rescue a group of baby Pokemon from a powerful and evil trainer. Sylveon could use its ribbons to distract the trainer while Pikachu zaps the cages open.
Their abilities can be combined in a rescue mission. For example, if there are Pokemon trapped in a cave. Sountland can sniff out the exact location of the trapped Pokemon from outside the cave. Flygon can then use its digging ability (if we assume it has some) along with its flying to enter the cave from above and clear the way for the rescue. This would show how their different abilities work together for a common goal.
Mimikyu has the ability to create illusions. In the fanfic, it could use this ability to protect Pikachu. For example, when a powerful and dangerous Pokemon attacks them, Mimikyu could create an illusion of a much stronger Pokemon to scare it away. Pikachu's electric abilities could be used to power up Mimikyu's illusions, making them more realistic. They could also use their combined skills to solve puzzles. Say there's a maze that is protected by electric barriers. Pikachu could discharge electricity to temporarily disable the barriers while Mimikyu creates an illusion of the correct path for them to follow.
Pikachu's electric abilities can be used for things like powering up machinery or zapping away small enemies. Glaceon's ice abilities can create bridges over water or freeze foes in their tracks. For example, if they are in a cave and need to cross a large gap, Glaceon can freeze the water below to make a path. And if there are flying pests bothering them, Pikachu can use Thunderbolt to scare them off.
Naruto's ninjutsu skills like the Rasengan could be combined with Tifa's martial arts and her Limit Breaks. For example, Naruto could use his shadow clones to surround an enemy while Tifa uses her physical strength to deliver a powerful blow. Then, Naruto could finish it off with a Rasengan enhanced by Tifa's energy or magic.
Neeko can shape - shift and Nidalee has her spear - throwing and hunting skills. In a story, Neeko could shift into a small animal to scout out an area, and then relay the information to Nidalee. Nidalee could then use her accurate spear - throwing to take out threats that Neeko has identified.
Slenderman's ability to be invisible or blend into the shadows could be combined with Enderman's teleportation. For example, Slenderman could hide in the shadows while Enderman teleports them both quickly away from danger. Or Enderman could teleport to a location and then Slenderman could use his spooky presence to create a diversion while Enderman steals an important item.
Well, Naruto's Rasengan combined with Itachi's Amaterasu could create a super - powerful attack. Konan could use her paper jutsu to guide or enhance the attack. For example, her paper could be used to direct the path of the Rasengan - Amaterasu combo towards the enemy.
Another way is for them to use their abilities for exploration. Rayquaza can fly high and far, carrying Latias with it. Latias can then use her psychic powers to sense things that are hidden or far away. For instance, they could be looking for a lost Pokémon or a special item. Rayquaza's speed and flight range combined with Latias' sensing abilities would make them a great team for such quests.