Just to clarify, I never said blast with scrapebox. Only AMR.
With a 3 year old ranked domain you can pretty much do whatever you want to do with it as far as blasting is concerned. However!!! not if you have a ton of shitty PLR, auto blog, scraped articles, etc. content. In this case proceed with extreme caution, as sites have been know to get deindexed.
You said in OP that you optimized for 1 keyword only. This looks completely unnatural, and the sort of thing only Internet Marketers would do. This is why I'm suggesting AMR blast with just naked URL in resource box. You need to deoptimize your backlinks!
Once you are regaining position, then you can try the other offered advise as far as optimizing other keywords.
Blast with original content you write. DO NOT SPIN. You are looking to just get your link out there and be accepted by as many article directories as possible. This has nothing to do with duplicate content! Since Panda Google wants to see QUALITY above all else. A well written article with your raw link in the bio syndicated across a thousand authority article depositories is SEO gold. Think about it. If an article is so relevant and well written that it is syndicated across the web, and it points back to your site, don't you think that site has something of value to offer to the searchers?
One last important point. After you write your article, publish and index it on your site first. Wait a couple of days, then blast. That way there is no doubt that you are the original publisher and AUTHORITY of this article/subject.
Also, just to really drive home how powerful this can be. If your article is written well enough, other publishers will very likely post it on their blogs/sites hopefully leaving you links intact. Some autoblogging software might auto distribute it for you. Guess what, you just got more backlinks from random ip's across the net.
Best of luck.