All Ultra Beasts and wormhole legendaries are huntable, but if you’re not willing to do more than 500 SRs for a shiny, then shinyhunting might not be for you, at least not SR hunts. All UBs/wormhole legendaries are either full or charm odds, so the best odds you could have hunting Pheromosa is 1/1365 if you have the Shiny Charm.
As far as catching it goes, just Roto-Catch + Beast Balls are going to be your best option, along with some of the standard paralysis/sleep + False Swipe stuff. Pheramosa’s natural moveset will be Triple Kick/Bug Buzz/Lunge/Me First, so luckily, it has no way of killing itself outside of Struggle, meaning that having a tank with resistances like Togekiss or Aegislash is all that’s needed in terms of special preparation.
Other than that, all I can say is be patient (watching something like YouTube while hunting really helps) and good luck!