On November 26th, Terrakion entered raids for the first time as a 5* boss. While it’s not the most highly anticipated boss, it’s absolutely the strongest of the Musketeers (Cobalion, Terrakion, and Virizion), and is an incredibly powerful Rock attacker in its own right.
How to Beat It
Use in Raids
As Rock-type attackers go, Terrakion is sure to make a name for itself. While it is outperformed by Rampardos in terms of DPS and DPS^3*TDO, it manages to beat Smack Down Tyranitar in both metrics. Given how difficult it is for most players to even get their hands on a good high level Rampardos, Terrakion absolutely shines there. It also does better than the upcoming Landorus and Gigalith! All of these factors combined means that Terrakion absolutely deserves a spot on your team!
The optimal moveset is Smack Down + Rock Slide. Close Combat’s mediocrity (combined with the lack of a Fighting-type fast move) makes Terrakion pretty useless as a Fighting-type attacker, and there are better Earthquake users.
Use in Trainer Battles
Best Moveset: Smack Down + Rock Slide and Close Combat
Sadly, despite the power of Smack Down + Rock Slide, Terrakion’s low bulk and menagerie of weaknesses really hamstring the Cavern Pokémon. It struggles in Ultra, and it struggles even more in Master. Given that it performs just as well against Raikou and Dialga (that is to say, "not very") with Close Combat, you can ignore Earthquake. You’ll probably just want to keep this one in your raid parties.
As for Rocket Battles (this is kinda trainer battles), Terrakion’s honestly not bad. Rock Slide’s speed relative to Stone Edge can give it a bit of a niche against the legendary birds, and its resistance to Bug makes it an alright lead against Arlo’s Fury Cutter Scyther (though only taking neutral from Air Slash is rough).
Cobalion? Useless. Virizion? Useless. Terrakion? Better than Tyranitar. After a nice relaxing couple weeks off from farming T5s, it's time to get back into it to get yourself a good Terrakion! Definitely something to consider maxing out, especially since it's not likely to meaningfully improve with the eventual advent of Sacred Sword!