Both Dragon Pulse and Outrage are good moves and it really depends on the situations your in, If you are looking to use Hydreigon last as a late Sweeper keep Outrage to finish foes off. If you are using it in early game keep Dragon Pulse. it all depends on its specific role.
I would say keep the moves you have at the moment. It allows you to abuse both his special and physical attack, while giving you coverage if you happen to be up against a steel type flamethrower fixes the problem since you won't be able to hit them hard with dragon type attacks. Also, I would suggest sticking with your special attacks, But if you have the chance to get in a Super effective physical go for it, especially since both of your physical attacks are stab boosted moves they should decent damage. Though I could sit here and explain that you are not using hydriegon to it's full potential, nature wise, and EV wise/stats but he should be fine for in game use. You just need to know what the Pokemon your attacking is built for, like gigalith have great defense, while being backed by sturdy, but not excluding their massive attack stat you should go for Dragon pulse, after stab and calculating in his mediocre Special defense you should be able to bring him down to sturdy in one shot. While I wouldn't be so sure of how good you would do if you decided to use a physical attack because his defense is surprising 130 base, which could give you troubles. So Just make sure to use the best move for your situation