Doing a nuzlocke, kinda really under leveled for Grant, I need to know where to grind to about 24-25.
Honestly, the yellow flowers on route 10 (just above town) have pretty decent-level wild Pokemon. Train with the Exp Share.
Basically, any route OUTSIDE of the town is really the only best place for grinding, or you could try inside of the gym. Grant's gym ranges from about the levels of 16-21 (i could be wrong), so really, there isn't any place for good grinding.