It's a solid set, but in all honesty Hydreigon loves taking out Tyranitar, 2HKOing Blissey and Heatran. You mentioned that you use Earth Power for Heatran, however the combination of Draco Meteor (that heatrans love switching into) + Superpower after SR damage take it out with a Mild or Rash nature. I noticed you have it slashed, I just really want to point out that while Hydreigon's SpA is stupid high, it's selling point is that of a wallbreaker. This is why it had nothing that could safely switch into it last generation.
Also surprisingly, Hydreigon really likes it's speed. It's not as slow as everyone suggests, while it is slow for a dragon. With your Evs however you hit a speed of 232. This leaves pesky pokes such as Max Speed Excadrill, Jolly Breloom, Mamoswine, Bisharp with Low Kick and hell, even Jolly Aegislash with Sacred Sword (which i've seen a couple times on the ladder for whatever reason...) being able to ouspeed and OHKO you, bar Excadrill. (Who comes pretty damn close.)
Basically Hydreigon really likes it's speed in OU and it's ability to wallbreak. That's my two cents.
Anyway here's my set. It's standard, but it's the Hydreigon I believe is designed to kill things and really make your opponents think about their switches.
Hydreigon (F) @ Life Orb
Ability: Levitate
EVs: 4 HP/ 252 SAtk / 252 Spd
Mild/Rash Nature
- Draco Meteor
- Fire Blast
- Superpower
- Roost