Steel is:
2x weak to fire
2x weak to fighting
2x weak to ground
it is resistant to:
Grass
Ice
Flying
Poison (immune, no effect)
Psychic
Bug
Rock
Ghost
Dragon
Dark
Steel
And the rest are just normal damage.
However, his pokemon a usually a combo of steel/something so you'll have to figure that out.
Wow,thanks Shibby.
I'm going to request this topic for a sticky.