This is a very solid Sandstorm team and the surprise value of Sandslash can really give your opponents trouble. Looking at your team, a major threat to it is Chlorophyll Venusaur
, the most commonly used sweeper on Sun teams. Under sunlight, Chlorophyll Venusaur outspeeds your entire team and can pull off a sweep with its standard coverage of Giga Drain and Hidden Power [Fire]. Garchomp and Tyranitar can take an attack but are cripple by Sleep Powder. For this threat, I recommend replacing Gastrodon with a Calm Mind Latias
with a moveset of Calm Mind / Recover / Dragon Pulse / Psyshock
. Calm Mind Latias is a great check to Sun teams with her massive Special Defense stat and resistance to Fire and Grass type attacks. She can take an attack from Venusaur and dispatch of it with a STAB Psyshock. Latias helps fill in the role of a Rain check that Gastrodon is currently filling as Latias herself is a great check to Rain teams with the ability to check the likes of Tentacruel and Keldeo easily. With Recover, Latias can heal off damage from the opponent and residual damage from status and accumulate more Calm Mind boosts. Dragon Pulse is an obligatory STAB that is quite powerful after a few boosts. Latias makes a great partner with Garchomp as the dual Dragon core can wear down each other's counters, opening a late game sweep for one or the other.
Now for some smaller changes. On Skarmory, I recommend a Specially Defensive spread
of 252 HP / 100 Def / 156 SDef
with a Careful nature. Skarmory already has a massive Defense stat even without much investment and can continue to wall most physical attackers. By investing in Special Defense, Skarmory can more easily take the likes of Tornadus-T's Hurricanes and Latios' Draco Meteors so he can serve as a great mixed wall for your team. Lastly, on Garchomp I recommend using Dragon Claw
instead of Outrage. Dragon Claw allows you to switch up moves and you won't have to worry about the confusion from Outrage that could possibly prevent Garchomp from sweeping.
Hope I helped and good luck with your team!