"You might consider using HP ice instead. If not then Hydreigon walls you all day. Also heatran is an issue, but that's as easy as earth power."
I am using Psychic + Hidden Power Fire because currently most of the most used Steel-types aren't weak to Ground but have a big weakness to Fire, and to have a super-effective attack against Fighting-types (Celebi's psychic is the only attack on my team that can hit pure Fighting-types for super-effective damage) but Earth Power + Hidden Power Ice is in fact a very good combination, I've suggested my friend to use this. I haven't tried to use Hidden Power Ice and I was very impressed to see Celebi, with a +2 Hidden Power Ice, OHKOing Dragonite with Multiscale intact!
252 +2 SpAtk Life Orb Celebi Hidden Power Ice vs 54 HP/0 SpDef Multiscale Dragonite: 98,21% - 116,07%
87,5% chance to OHKO
This is assuming that the Dragonite is a standard set; you have 87,5% chance to OHKO. The bulky variant has only 12,5% chance to be OHKOed; and the Multiscale Abuse set isn't going to be OHKOed anytime soon unless you get a critical hit; however, all these variants are guaranteed to fall after Stealth Rock damage.