I think you may find it interesting - I've managed to negative SEO my own website and nuke my own rankings.
I've observed with 2 of my clients so threat is real.
Did you ever had any similar cases?
There was one discussion here about that already:
There is a lot of movement in rankings today. Sites that jump in rankings are mostly Penguin 3.0 sites.
I wrote just a quick Penguin 3.0 summary here https://goralewicz.co/google-penguin-3-0/.
What is interesting, Penguin 3.0 still is susceptible to Negative SEO attacks. I am helping out some negative SEO victims (some of u may know Kumar and his directories) and some of the websites were...
If you guys are doing a lot of link processing, or any work that requires some help from the "black force", there is another tutorial about ScrapeBox for white hat SEO. I think it is quite popular topic recently:
I think you may find it quite interesting.
How to create right link building strategy. Tutorial is based on on-page structure and is showing best practices to merge off-page and on-page efforts...
I don't know if there are many people here doing link audits ;) If so, that is a super easy technique to get more links from your Google Webmaster Tools account http://www.linkresearchtools.com/case-studies/get-more-links-google-webmaster-tools-gwt/
If you got domains with subfolders...
This is a White Hat SEO part of this forum :)
Depending on where are the links from, it would be a good idea to outreach those people linking to your competition and for example create some good content for them. There are tons of options, but commenting is definitely a 2009 solution...