I'm sorry to say, but that is a truly terrible moveset. Ya did a few things right, but that sorta plays to his weaknesses. He has pathetic sp.atk, so don't even bother with Giga Drain. Instead, here's a few movesets of mine
Torterra-Leftovers
Nature: Impish/Adamant
Earthquake
Wood Hammer/Seed Bomb
Stone Edge/Rock Slide
Synthesis/Leech Seed
This plays to his strenghts-being a wall. He has awsome attack, and great HP, so this really works. Earthquake for STAB. Now, you could go with Wood Hammer, my personal choice, but it has a pretty nasty recoil. Seed Bomb is much weaker, but no recoil. You also have to breed for Seed Bomb. If you don't want a grass type, you can just stick with Stealth Rock.
Stone Edge/Rock Slide, along with Earthquake, covers a lot of types. Stone Edge is powerful, but inaccurate, and only 5 PP. Rock Slide is m uch less powerful, but 15 PP and more accurate.
Synthesis can help you recover from your own Wood Hammers, and adds to his staying power. But with all the Tyranitars, and other weather effects, it may be useless. Now Leech Seed is great, along with Leftovers, but if you use Wood Hammer, it won't help you recover as much. If you choose Seed Bomb, definetly use Leec Seed.
You gotta learn to take advantage of your pokemons great strenghts. In Torterra's case, high HP, def, & atk.
Hope this helps :)
EDIT: Not you, Key, your set is also fine.