Fire Water Grass cores work really nice for their ability to cover each others weaknesses. VoltTurn is also super prominent in Gen 5, so I'd suggest putting Rotom-Wash on your team as a Volt Switcher. (Infernape as the U-turner) If you aren't satisfied with Rotom-Wash, you can replace it with Gyarados as another water Pokemon that can cover Ground weaknesses.
The grass Pokemon can easily be Ferrothorn Ferrothorn just helps wall a huge chunk of the metagame. Ferrothorn stacks typings with the later Hydreigon pairs, so be careful of that, but if you really want to use Celebi with its 4x bug weakness, it could be a good Stealth Rocker.
Even if Hydreigon has coverage for Ferrothorn, you still might want a steel trapper to make sure the steel type you are trying to take down cannot switch. Magnezone is the best one for this with it's high special attack and better offensive typing. Hidden Power Fire can help take out Ferrothorn, Forretress, and Scizor easily.
Hydreigon struggles against special walls like Blissey and Chansey. Mamoswine is a really good physical attacker that doesn't stack typings with your team and can be an alternative Stealth Rocker, or just fire off powerful moves like Icicle Crash, Earthquake, and Superpower.
Both of the Hydreigon pairs are weak to fighting, so I'm going to suggest a fighting resist or immunity. Offensive checks to fighting types are Gengar, which can switch in to a fighting move, or Gliscor, which stacks typings, but can reliably check most of the Fighting types bar Keldeo, which can be checked with Ferrothorn to an extent.
Hope this helped!