Does any one know how critical rate in GSC take account stat boosts / drops?
RBY: CH ignores all offensive / defensive boosts / drops
ADV onwards: CH ignores offensive drops and defensive boosts. For instance vs a -1 Atk Mon A vs +2 Def Mon B, a CH would calculate as if Mon A had +0 Atk and Mon B had +0 Def. If Mon A had +2 Atk, or if Mon B had -1 Def, then those stages will not be ignored.
I believe CH in GSC ignores both drops in offensive and defensive stat, but i'm not quite certain.