After defeating the Elite Four and finishing the main storyline, you will be able to start battling and catching the Ultra Beasts through the Looker Storyline. The second Ultra Beast you are set to capture is Pheromosa. Like all Ultra Beasts, Pheromosa has some pretty incredible Stats. It's more of a Sweeper Pokemon, with high stats in both Attack, Special Attack and Speed, but it's not bulky at all with downright terrible defensive stats. Luckily it's a Bug/Fighting Type, which means a player can disable it with paralysis.
Here Are Some Tips For Catching Pheromosa:
Like many of the other Ultra Beasts, you are set to catch more than one Pheromosa, four to be exact. It can be found randomly in Verdant Cavern. Since it's at Level 60, a player can easily put a Level 60 Pokemon at the lead of their party and use a Repel to quickly find Pheromosa in this area. A player will know they are near this Ultra Beast by a change in the music and a not-so-far off cry from it.
Like most Legendary Pokemon, it's best to try to paralyze Pheromosa first thing so you can always go first, especially since Pheromosa is one of the fastest Pokemon in all of The Alola Region. Right as the Battle begins, Pheromosa's Speed will increase by two stages, so make sure to paralyze it first thing. From there, try to lower Pheromosa's HP as low as you possibly can. Moves like False Swipe and Hold Back work great when it has low HP, ensuring to leave at least one HP, especially since it has horrible defensive Stats and one hit besides these move could easily defeat it.
But keep track the more Pokemon Pheromosa knocks out on your team, as its Highest Stat will increase every Time it does so because of its Beast Boost Ability. Because of this, like all the other Ultra Beasts, it's best to try to capture it as quickly as possible with the Beast Ball, since it has a x5 Catch Rate with Ultra Beasts. But don't be afraid to use Ultra Balls or either Nest Balls on Pheromosa, as they both work well on it.
And don't forget to ALWAYS save before trying to find Pheromosa. It's a safety precaution, one that has worked since the first generation of Pokemon.