In Pokemon, there are 6 stats that can be affected by characteristics: Attack, Special Attack, Defense, Special Defense, HP, and Speed. Most Pokemon have an even distribution of these stats, but there are a select few that are incredibly unbalanced. This means that in some areas, they are the best, while in others, they fall flat. Deoxys is one of these Pokemon, depending on what form it takes. In Attack form, its attack and special attack increase while its defense and special defense fall. On its own, this doesn’t make Deoxys that much of a threat, but take into account that Deoxys has a high base speed, so most Pokemon would be one shotted by its moves. Only Pokemon with higher base speeds like Electrode and Ninjask or Pokemon with priority attacks would be able to stop it. It wouldn’t be too tough, considering that its defense and special defense is lower than Whismur’s. But also realize that some Pokemon can be quite bulky, and unless your move is STAB, it might not one shot it. Since Deoxys is a Psychic type, it can’t do much against Steel types, and it really can’t do anything against Dark types. Sharpedo might be a good Pokemon to combat Deoxys-Attack, because although it isn’t faster than Deoxys, it does get Speed Boost as a hidden ability, which raises its speed by one stage every turn. It also gets access to stalling moves like Protect. It also has a Mega Evolution with Strong Jaw. Combine that with a STAB Crunch and Deoxys can easily go down in one hit. Ninjask would also be good because Bug is super effective against Psychic. If you’re looking for another Pokemon with Deoxys-Attack’s stat distribution, you’re in luck.
Pheromosa, the Lissosome Pokemon, is an Ultra Beast. It is Bug/Fighting type. Although its attack and special attack are not as high as Deoxys-Attack’s, it does have a higher speed. It is only outsped by Ninjask and Deoxys-Speed. Unfortunately, both of them have typings that Pheromosa might have trouble with, especially Ninjask. However, it is possible to outspeed them both, making Pheromosa the fastest Pokemon ever. All of the Ultra Beasts have the ability Beast Boost, which acts like Moxie except it raises the Pokemon’s best stat. Moxie raises the user’s attack by one stage every time it defeats a Pokemon. And for Pheromosa, its highest stat is its speed. Although the Ultra Beasts have better stats to raise (Special Defense, Attack, and Special Attack), speed is an important factor in a battle. So all Pheromosa would have to do is beat one Pokemon (which, depending on what it is, shouldn’t be too tough) and its speed would be higher than Deoxys-Speed’s.
With all this in mind, how would one combat a Pheromosa? That’s not so simple. There are no increased priority moves that would be super effective against Pheromosa, and although its defense is lower than Chinchou’s and its special defense is lower than Bounsweet’s, its defense and special defense are higher than Deoxys-Attack’s. Plus, there are increased priority moves such as Shadow Sneak and Sucker Punch which could easily take down Deoxys-Attack in one hit. Your best bet is to somehow outspeed it. One method would be to paralyze it, and paralysis will last the whole battle for Pheromosa, bringing down its speed. Klefki would be a good Pokemon to use for this as it has access to Thunder Wave and its regular ability is Prankster, which gives priority to status moves. From there, if your Klefki survived its turn, you could take it down with a Fairy type move. If not, you could use another Fairy type, or Fire, Psychic, or Flying type. Some Psychic types like Reuniclus or Beheeyem that are slower would not be advised because they are slow. Even though Pheromosa would be paralyzed, it can still outspeed you if you’re slow enough. It could take you down easily with a STAB Bug type move. Besides that, taking down Pheromosa should be easy enough.