Hippowdon, when both physically and specially defensive, cannot do a thing against bulky Water or Grass-types such as Rotom-W and Celebi, who in return can easily KO Hippowdon with a STAB super effective move. Blissey serves as a good partner as it can wall and Toxic both of these, while also providing Aromatherapy, and Wish support. Rapid Spin users work well as Hippowdon is vulnerable to all the hazards, so a spinner like Tentacruel or Forretress work well. As for the Life Orb Tank set, Tyranitar is mandatory when using this set to get out the full potential. While sharing many common weaknesses, it gives Hippowdon Stealth Rock and Sandstorm support which make it that much better of a sweeper. This Hippowdon also appreciates Rotom-W as a partner due to it's ability to beat Skarmory and Gliscor easily. Spinners work as per usual, as this Hippowdon gets worn down relatively fast between Life Orb and hazards. Wish support is also very helpful to make sure Hippowdon comes in at high HP to keep his bulk at a respectable amount. Finally, hazards are very helpful, as Hippowdon, like every offensive Pokemon, appreciates the enemy's team being worn down.
As this is a good description from smogon and such I would like to think that a Mega Gyarados would be a great partner to it. With great attacking stats it would be great with some Waterfall, Ice Fang and Crunch to provide help. If it's in-gameuse something like mega Venusaur that resists it weaknesses and got a very good move pool like Giga Drain and Sludge Bomb with STAB.
As you already got a mega I suggest you could use Excadrill. It's a awesome Pokemon and was Uber. Good bulk and gets the boost in the sandstorm and could help your team. It got both rapid spin and stealt rock. EdgeQuake combo also includes. Stalls both dragons and fairys.