This is a great team, props to you for using Sandslash which tends to be fairly unpopular in today's Rain metagame. However, your team is heavily threatened by Swords Dance Scizor. While it's most commonly seen as a Choice Band version, Swords Dance Scizor is still fairly popular and very threatning because not a lot of people are well prepared to take it on. Scizor can very easily set up on your Ferrothorn, which lacks a move that can beat it or even force it out, or your Tyranitar which Scizor easily scares out, and then proceed to destroy the rest of your team with the combination of Bullet Punch / Bug Bite / Superpower. Bug Bite takes care of Slowbro, Ferrothorn and Tyranitar while Bullet Punch takes out your faster Pokemon, namely Salamence and Sandslash. Your own Scizor is outsped by the most common Swords Dance version and is OHKOed by +2 Superpower. One change you could make to fix that weakness is replace your Scizor with an Heatran. Heatran can outspeed and OHKO Scizor with Fire Blast, while having a x4 resist to Bullet Punch. You'll have to be careful not to switch into Superpower though. Besides, Scizor and Heatran have pretty similar defensive coverage, and while Scizor is much better at checking Latios, you have Tyranitar for that. Slowbro can also make up for the loss of a Terrakion check, and you can still beat it with Sandslash. Bullet Punch and U-turn are pretty big losses but it gives you an alternate check to Volcarona if Sun is up, which is always amazing, while also preparing you against most Scizor variants.
You should also use Power Whip over Protect on Ferrothorn. While Protect allows Ferrothorn to recover health with Leech Seed, Ferrothorn needs to be threatning to all the Water types it walls. Otherwise, stuff like Rotom-W could really be tempted to stay in to Will-O-Wisp Ferrothorn which really cripples it badly, while if it sees Power Whip it might be really afraid and try to switch out. Since your team doesn't really handle Rotom-W that well it would be a very worthwile addition. Starmie could also just Recover the damage and be alright. In short, Power Whip makes Ferrothorn a danger to Water types and really reduce the momentum they have by forcing them to switch out.
Cool team, here's the set you should use:
@ Air Balloon
EVs: 4 HP / 252 SpA / 252 Spe
-Hidden Power Ice
Edit: Yeah should've explained why you should use Toxic on Heatran. Basically, two of Heatran's most common switch ins, Politoed and Rotom-W, really hate Toxic. This is especially useful against Politoed since it's a good way to win the weather war if you catch it on the switch with Toxic, and you seem to rely on Tyranitar's Sand Stream for your team to work better, which is why you should use Toxic.