You would have to do it with two UILabels. You would set the first label to be all of the text excpt the \%%, and get the size of that label using sizeWithFont: on the text that goes in that label. Then set the second label to start at the end of that label's frame.
So, it would look something like this, changing the coordinates based on where you want the labels:
NSString *criticsScore = [NSString stringWithFormat:@"%@",[dict objectForKey:@"critics_score"]];
NSString *str2 = @"\%%";
UIFont *criticsFont = [UIFont systemFontOfSize:[UIFont systemFontSize]];
UIFont *font2 = [UIFont systemFontOfSize:12.0];
//Get the sizes of each text string based on their font sizes.
CGSize criticsSize = [criticsScore sizeWithFont:criticsFont];
CGSize size2 = [str2 sizeWithFont:font2];
int x = 0;
int y = 0;
//The first label will start at whatever x and y are.
UILabel *label1 = [[UILabel alloc] initWithFrame:CGRectMake(x,y,criticsSize.width,criticsSize.height)];
[label1 setFont:criticsFont];
//Create a second label with the x starting at x+criticsSize.width;
//The y will start at y+criticsSize.height-size2.height, so that it will be aligned with the bottom.
//Change these to align it differently.
UILabel *label2 = [[UILabel alloc] initWithFrame:CGRectMake(x+criticsSize.width,y+criticsSize.height-size2.height,size2.width,size2.height)];
[label2 setFont:font2];
[self.view addSubview:label1];
[self.view addSubview:label2];