Just pointing that if you REALLY want to beat specially defensive Tyranitar (and I hope there's not many of those left), the most sensible move is Grass Knot to achieve a 2HKO after SR, not Rain Dance + Water Gem. Rain Dance alone has some merit but Water Gem is wasting every other boosted attack you could've had. Here's the facts-
-Life Orb Grass knot will usually kill max 252 HP / 252 SpD Careful TTar after SR, 2 slightly below average rolls will get the job done (if it gets the 9th lowest possibility out of 16 both times it does it). Specs Latios obviously doesn't have many problems.
-On a ridiculous 0 HP / 252 SpD CB Tar Life Orb Surf already gets the job done almost every time (12th lowest roll both times does it). 252 HP / 0 SpD Tar is gone every time.
-Scarf Tar can stomp you on the switch if you try to Rain Dance and then Water Gem Surf, and you should be way way way more worried about this set than Specially Defensive Tar. If you want to beat this still Life Orb HP Fighting has a 6/16 chance to OHKO after SR and Specs guarantees it. That is your only option, not Rain Dance + Water Gem. If you want to beat it without HP Fighting you better have Dugtrio or something like Terrakion / Garchomp to set up on the Pursuit.
Not only does Grass Knot trip Specially Defensive Tar, which again you should not be worrying much about, it also lets you beat Jellicent, Hippowdon more easily and OHKO Donphan once Sturdy is gone (I don't know why people are using Donphan so much but they are). HP Fighting does OK vs natt, but Specs can't even 2HKO after SR if it's max Special Defense and if it's lighter, you just locked yourself into HP Fighting against a Rain team which is just asking for trouble with or without Torn-T. If you use HP Fighting don't expect to hit anything but ScarfTar with it.