I'm sure you're aware of this, but another means to planetary elimination is a particle accelerator accident. According to Bostrom and Tegmark,

There are three ways that a particle accelerator could hypothetically end our world. It could spawn a planet-swallowing black hole; it could create strangelets, weird matter that alters all matter around it; or it could rip apart the structure of space and change the laws of physics. Researchers concluded that cosmic rays—natural high-energy particles—are much more likely to cause such disasters than a particle experiment is.

Read more.

That would, indeed, be a Class X apocalypse. Nice to know that they aren't all off in the distant future.

Excellent apocalypse scale.

Some of your readers might be interested in this examination of how its levels have been depicted in movies:
http://www.futuristmovies.com/blog/index.php/2007/01/24/apocalypse-in-7-not-so-easy-steps/

Interesting news on this topic recently:

http://space.newscientist.com/article/dn11207-asteroid-threat-demands-response-experts-warn.html