definitely PBNs, but a little risky
Risky is only low SEO knowledge, especially how Google detecting PBNs.
But more risky is following fake SEO gurus, who already have all (or part) their PBN banned.
And until today they don't found the answer "why", from the reason above.
@cosmo89
PBN'S will definitively have more impact on your article. More efficient with ranking. From all the BlackHat SEO techniques, PBNs are probably the most efficient. The better the expired domain names you purchase, the faster you'll see the results.
Best regards.