Garchomp
I'd suggest Swords Dance over Rock Slide. Or, you can use Stone Edge instead. Its not necessary, but I think you should consider Scale Shot over Dragon Claw. EVs, item, and ability are great though.
Gliscor
Gliscor is good.
Hippowdon
Hippowdon is more of a defensive Pokemon. You should use Leftovers or Rocky Helmet over Sitrus Berry. Adamant Nature and 252 Attack EVs should be replaced by 252 Defense EVs and Careful. You should also replace Stone Edge with Slack Off.
Excadrill
Salac Berry isn't needed on Sand Rush Excadrill, or Excadrill in general, except maybe, just maybe, on Substitute Mold Breaker sets. However, since you have Hippowdon, Mold Breaker isn't needed. You can either keep Rock Slide or replace it with Swords Dance, it's up to you.
Mega Steelix
Mega Steelix looks fine, except you should tweak the EVs to make it more physically defensive, since Body Press is the main move on the set. 252 HP / 252 Def / 4 Atk or 4 SpD should work, and you should give it a Impish Nature. Or, since you have Iron Defense, you can go 252 HP / 4 Def / 252 SpD and a Careful Nature.
Landorus-Therian
Expert Belt isn't that good on Lando. You should use a Choice Scarf set, or keep Swords Dance and use Leftovers. Rock Slide should be replaced by Stone Edge. If going Choice Scarf, use U-turn instead of Swords Dance.
