Poliwrath resists Fighting? Neither Water nor Fighting resists Fighting...
I would like to suggest Physically Defensive Poliwrath. Not only it resists both of Terrakion's STABs, but you also get a decent Scizor and rain check in general as well as a decent Tyranitar counter.
Poliwrath isn't a good OU Pokemon, I have tried it myself a few times. Even with Swift Swim on a Rain Dance (NOT Drizzle) team.
Sorry if I sound rude, but you should test some Pokemons before really bashing it stat-wise.
The most effective Terrakion check for me has been Gliscor, so I'm just gonna second the Gliscor recommendation on that one.
"Whether Castform the Weather Pokemon can weather the weather is dependent on whether the weather that it's weathering is the proper weather for it to weather. Whether the opponent can weather the weather is also subject to whether the weather that it's weathering is the right weather to weather. Whether Castform can weather the opponent is once again dictated by the weather and whether that weather is the favorable weather for weathering. But most importantly, you should know whether there will be weather to weather rather than what weather they will be weathering and whether they can weather that weather."
- Smogon's Castform analysis