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I don't think that Rock/Ground is a bad typing.Adding Fire,Flying and Normal resistance makes Ground Types even more bulkier.
As for the worst combination I would go to Bug/Grass and Ice/Grass.Because Looking alone on the types that they are weak to is already pain for me.Plus the two notable...
Blue-Classic
Lance/Red-Another Classic theme.Plus I love the opening piano on HGSS version
Steven/Wallace-Motivating and in the same time EPIC
Cynthia-Her theme resembles her Garchomp.CHALLENGING
Alder-Resembles his humbleness and simplicity as a Champion.He is like Pope Francis in Pokemon World...