Just saying: These things are NOT viable:
- Mixed Chomp - Too low Sp Atk. If you really want the surprise element, use a special set.
- Giga Impact - Totally overpowered by STAB moves who don't give a free turn away: Earthquace (150 base) or Outrage (180 base)
- Dropping Fire Fang (Or Flamethrower/Fire Blast) for a non STAB move - The coverage for Ferrothorn, Forretress, Skarmory... (Long list) is too important to drop it for other coverage
- Dragon Tail in a Scarf set - Never use negative priority in a speed set
- Dig - Never give away free turns for setup or countering
- No STAB moves - Never go without Garchomps monsterous STAB moves. Their power and usefulness isn't matched by any other options Garchomp has.