The first set looks pretty good, but you really probably wont want to use Giga Impact. It looks good at first, but that turn it takes to recharge is the turn they'll revenge-kill him, so it's only really useful as a last-ditch effort kinda thing, and nothing's even weak to it. I also can't find how a Haxorus would learn Dragon Dance, I assume you mean Swords Dance(and if you didn't, do, it's a wonderful attack on him).
Never use a nature like modest for mixed attacking, because it lowers your attack stat, and haxorus's Special Attack is useless.
thats the first good review on one of my pokemon suggestions all day.
Have any ideas what to replace giga impact with?
then check out this
Octillery Modest
EV's 152 Speed so i can get a good hit in first
100 in Hp so i can take some hits even though his not to good in it
252 SP ATK to blow him//her out
MOVESET
Flamethrower
Water Spout
Energy Ball
Focus Energy
I know he should be a sweeper with a base atk at 105 but ima breed for highest stat SP ATK but i thought it would be an ok sweep for OU Battles cus no one will expect an octillery.