The best way to hit Sub Terrakion is to get 2 layers of TS before it comes in. A little rapid spin support, and Sub Terrakion is deadly.
It is walled by a few things however, but usually a team carries the standard threats to Terrakion; "Scizor", "Gliscor", "T-Waver's", "Rotom-W whom is scarf", "Garyados"
Lets see....
Scizor<Sub
Gliscor<Sub + SD
T-Wave<Sub
Scarf Pokemon<Sub
Garyados<Sub [Stone Edge?]
Sure, you might say Garados is a bad counter, but with Intimidate, bounce, and substitute, its really annoying for the standard Terrakion.
There are a few, but these are usually what is ran to check/counter Terrakion, and this set just can do so much shit to them.
Psychic types do become a problem, which is why Scizor is an ideal partner [BP on
Lati@s], as when you make sets, as I often see on Smogon Analysis, there are reccommended support.
Scizor + Hazards + Wish are like god support for Sub Terrakion, thought I would mention it :)