Any team having trouble with Terrakion would do well to include a Focus Sash Dugtrio in it's lineup.
Lead Terrakion is completely beaten my Dugtrio, being OHKO'd by Dugtrio's Reversal if it opts to CC and 2HKO'd by Earthquake, even with a Jolly nature. At the end of the day, the result a dead quadpedal, a 1HP Dugtrio and either a) Rocks on both sides or b) Rocks on neither side. In fact, if Dug doesn't want to set up hazards, it survives with a Focus Sash intact, ready to hit most switch-ins decently hard or guarantee a Stealth Rocks setup next turn. Dugtrio in turn can proceed to hit something for reasonable damage if rocks are up, or switch out to trap something like un-scarfed/Balloon'd Heatran later on if rocks are not present.
Arena Trap also allows Dugtrio to shut down any Terrakion attempting a sweep lacking Quick Attack, most notably the fearsome Choice and Double Dance variants (but requires Reversal AND Sucker Punch to do so, given Sucker Punch's low power).