There are many unconventional methods but I can not reveal all. I want to share just one method I use - Search for a competitive keyword in your niche. The top ranking sites for this keyword are all powerful authority sites. We are not looking for them. You need to scroll down - find the niche sites ranking in 20+ positions. We are looking for sites which are not very powerful, not very old but ranking for a few thousand keywords, preferably 100+ keywords in the top 3. Then you can take those keywords and create your content. Now if you can find many small sites like this, you can get thousands of easy to rank keywords very fast. That's it.
I used to search for free websites like BlogSpot, WordPress etc. which are ranking for many keywords. These sites are very easy to outrank.