After defeating Ultra Necrozma, you will be permitted by the Ultra Recon Squad to continuing using there Legendary Pokemon to travel through Ultra Space. As you do this very thing, there is a chance you will travel to the Ultra Desert and find Pheromosa. Like all Ultra Beasts, Pheromosa has some pretty incredible stats. It's more of a Sweeper Pokemon, with high stats in Attack, Special Attack and Speed, but it's not bulky at all with its downright terrible defensive stats.
You will always find and battle Pheromosa at Level 60. The first thing you should do to limit Pheromosa is paralyze it with the move Thunder Wave, almost guaranteeing your Pokemon will always move first and making it much easier to capture. Fire Type Pokemon fair well against Pheromosa, but make sure not to hit it too hard since it's extremely frail defensively. Try to lower Pheromosa's HP as low as you possibly can. A move like False Swipe works great when Pheromosa has low HP, ensuring to leave it with least one HP. If you have the Rotom Power that makes it easier to catch a Wild Pokemon, then you should definitely consider using it here as well.
But do keep track, as the more Pokemon Pheromosa knocks out on your team, 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 Beast Balls, as they have a x5 Catch Rate when used on Ultra Beasts. 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.