I have a new website and my competition is so hard among over than 300 websites on only one keyword. My competitors are using backlink strategy through posting their links in blog comments. I heard that it is not legal, but they are dominating the organic search by the this strategy. My question is: How can I use the backlink strategy to make my website get higher?
Typical comments links might be no follow links so it probably isn't what make your competitor ranks higher than you.
You said it's illegal, no it's not illegal. backlinking is completely legal on a legal(justice) point of view. Artificial linking is just againt the Google webmaster guidelines.
But if you want to perform in hight competitive keywords, you have to gain more and more backlinks. It can be done by buying them, obtaining them with high relative content on less competitive keyword (linkbainting), using private blog network and much more other technics.
What you have to keep in mind is : it's legal, you just have to masterize how to do it before doing it on you main website activity, if not, hire someone who know it well.