This core basically compresses a typical Fire/Water/Grass core into just two mons, with Pyroak taking care of Grass, Fairy, Electric and Steel types, while Arghonaut deals with Fire, Water, Rock and Dark. Arghonaut uses a standard Specially defensive EV spread, to reduce the damage from Volkraken and Greninja, while Pyroak invests fully on the physical defense, something unusual in the current meta, as Specially Defensive variants are much more common. However, it still able to check the likes of Krilowatt and Koko, and complements Specially Defensive Arghonaut better. This core also packs both Stealth Rock and Spikes, so they can be used to rack up residual damage against the opponent, which is great in defensive and offensive team alike.
All that being said, there are a lot of ways to break this core, such as Tornadus-T, Tapu Lele and, M-Medicham , just to name a few. Therefore, to make this pair truly shine, you still need some support from other teammates. Zapdos, Mew and Kitsunoh are some of the best options available for this job, but there are many more possibilities, both defensively and offensively to complement this pair.