"Iphone 6 battery replacement" the word on the home page is difficult?

Hello, I have a website is the theme of mobile phone battery, http://www.mpbatt.com I would like to optimize the "iphone 6 battery replacement" the word, but I found the word on Google search volume is only 200 million, but it is in the Google trends score To reach 100 or so, I am now not sure the difficulty of optimizing the word is not great? So, would you like to ask me to see if the word is worth optimizing? Is it highly competitive?
If it can be optimized, where can I start from the positive? What kind of good advice to me!
I am very grateful to this!