Scarf Hydreigon is always Timid, bu you can also use a LO set:
Code:
Hydreigon @ Life Orb
Ability: Levitate
EVs: 4 HP / 252 SpA / 252 Spe
Modest Nature
- Draco Meteor
- Flash Cannon
- Dark Pulse
- Fire Blast
And for Garchomp, Jolly can be used for Mega or normal sets!
Examples:
Code:
Garchomp @ Garchompite
Ability: Rough Skin
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Swords Dance
- Earthquake
- Dragon Claw
- Fire Blast
OR
Code:
Garchomp @ Lum Berry/Focus Sash
Ability: Rough Skin
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Stealth Rock
- Earthquake
- Dragon Claw/Outrage
- Swords Dance/Fire Blast
Assuming he's going to be playing OU. Both can be used for DOUbles/VGC, but those would have different sets.