Terrakion is definitely one of those Pokemon that's pretty difficult to prepare for, due to how many items and sets it can run: Focus Sash, Life Orb, Fighting Gem, the Beat Up set, etc. Rock Slide is almost always a given on Terra, but choosing between Sacred Sword and Close Combat can be difficult: Sacred Sword hits past stat boosts but is less powerful, while Close Combat can nuke a surprising amount of things but leaves Terrakion more frail. Protect is almost always on every set too (sometimes Quick Guard), but the final moveslot can be variable: Swords Dance, X-Scissor (my preferred choice), Earthquake, etc. The metagame shifts has actually given Terrakion a better time too, as most Thundurus don't run max speed anymore, letting Terrakion smack them hard with Rock Slide.
Intimidate doesn't cripple Terrakion as much as other physical attackers, but is still a big problem he has. Scizor and Metagross are pretty good counters as Bullet Punch hurts Terrakion very much. Cresselia is also another obvious counter.
Anyways, those are my thoughts. Good Pokemon.