Hey. I like the structure of your team, and how it focuses around the excellent combo of Reuniclus and Terrakion. However, your team is very weak to opposing Latios that use Life Orb over Choice Specs. Your team gives Latios multiple opportunities to switch in and run through it. To deal with it, I'd recommend replacing Xatu with a
Jirachi. You already have multiple means of checking Breloom and Amoonguss, so Xatu is not as needed. Sure Magic Bounce is nice, but having a reliable check to Latios is even nicer. Jirachi can also lay down screens if your team so demands it. Also, to deal with powerful physical attackers, I'd recommend making your Hippowdon a
physically defensive variant. This lets you cover threats such as Terrakion and Landorus better. I'd also recommend using
Rock Gem over Leftovers on Terrakion, so you can score the OHKO on several notable physical walls, such as Gliscor and Skarmory, with Stone Edge.
Sets:
Jirachi @ Leftovers | Serene Grace
Careful Nature | 252 HP / 4 Def / 252 SpD
Iron Head | Body Slam | Wish | U-turn
Jirachi @ Light Clay | Serene Grace
Careful Nature | 252 HP / 100 Def / 156 SpD
Reflect | Light Screen | Wish | U-turn
Nitpicks:
252 HP / 4 Def / 252 SpD ----> 252 HP / 232 Def / 24 SpD
Leftovers ----> Rock Gem
I listed out two Jirachi sets: a specially defensive variant, and a Dual Screens variant. Try out the one that you feel will suit your team the most. Of course, feel free to mess around with the EVs of the Dual Screens variant until you feel like they are the best they can be. Wish support from Jirachi can keep your team especially healthy. Nice team, and good luck!