This is a pretty solid team, and most of its weaknesses have been outlined above. At the moment, SubCM Jirachi looks like the most threatening Pokemon for you to face. An Agility Thundurus-T can also be deadly if it manages +2, with Latias and Rotom-W being your only ways to reliably stop it. Unfortunately both take 60+% from its coverage moves, in Rotom-W's case possibly 80%, so keeping them in top health should be a priority when playing a Thundurus. In my opinion, this threat doesn't warrant a change on your team, as Latias is a pretty decent check, but I just wanted to make sure you were aware of its potential sweeping capabilities. Like other users have said, SubCM Jirachi can setup on a large portion of your team and deal serious damage. Another threat, albeit a much less minor threat, is SD Toxicroak. It can put a lot of pressure on you, with Scizor being your only reliable way to kill it and your second best shot is trying to stall Sucker Punch's with Starmie. Otherwise a faster SD set with Ice Punch looks troublesome to face.
Ok so at the moment your team has really good synergy, so changing a Pokemon would probably be more hazardous than helpful. For this reason, most of my changes will be minor moveset changes. First off, I think you should put Earthquake > Rock Slide on Terrakion. I know you just recently made this change, but Earthquake does have a special niche on this team. Earthquake not only helps to hit Jirachi harder, but also does reliable damage to Toxicroak. By using Earthquake you have another reliable way to hit Toxicroak for major damage, limiting his sweeping capabilities somewhat. I would also recommend taking CTC's suggestion of defensive Heatran. This will give you a fairly solid answer to Jirachi, while giving yourself more bulk overall.
Anyways, cool team gl.