Well, Relaxed raise pokemon's defense while lower their speed. I'd prefer to breed again with Adamant(+Atk,-Sp.Atk) or Lonely (+Atk,-Def) Ditto. Use chain breeding also. You don't want your Flygon to get hit before it hit its enemy,do you?
Really, the only Sp.Attack you should use for Flygon is Flamethrower/Fire Blast. Dragon Breath is weak... but Dragon Claw is plausible if all else fails. :/
If you want a physical pokemon, don't use a mixed set. That is just messed up. This is my opinion:
Flygon@CB
Hasty (He has no physical weaknesses)
EV's: 252 Att./Speed, 6HP
EQ DC
Fire Blast (Equal to Flameclaw)
Iron Tail (For everything neutral to Dragonblast and that EQ can't hit)