blur edges of one photo inside of another?

I'm practicing with a green screen for the first time.  In addition, I'm trying to change the scale of my barbie in relation to the background.  I'm not sure how to smooth out the one photo into the other.  I've played with the green screen elements but I want barbie crisp but the mountain to blend into the other mountain.  I've tried with gaussian blur and with opacity. 

 

Any suggestions?  Images attached. 

 

thank you!