So I'm going up in a pokemon battle in X and Y. Each person is assigned a type, and your team (Of 3). We can only use Berries and Leftovers.
My assigned Type is DARK, while my opponent is assigned STEEL.

My Pokemon Are:
Protean Gredinja (5iv)
Illusion Zoroark (5iv)
Chlorophyll Shiftry (5iv)

I know his pokemon will most likely be
Aggron (5iv)
Metagross (6iv)
Magnezone (4iv)

What do you recommend I do to beat him?