After messing around with Attumen for a while, I figured out that he and his horse, Midnight, are not linked. That is, they do not both engage in combat at the same time. In order to take advantage of this, I simply pulled Midnight back just before he called for his master (at 95%). Because of the level difference between Attumen and my pet, it is possible to avoid proximity aggro. If done correctly, Attumen will not engage, making it much easier to keep my pet alive. For non-hunter classes, I'd imagine you could employ the same exploit by running away as Midnight drops to 95%.
After Midnight drops to a certain percent, he merges with Attumen and drops combat but doesn't reset. If I wanted to, I could have sat and regened a bit before re-engaging. From that point on, it's smooth sailing as he doesn't hit any harder than Midnight alone did. The only thing to keep in mind is to stay within 10 yards of him or else he charges you, as you can see when I stood too far out in the video and was nearly killed.
As for why you'd want to solo Attumen, he has a 1% chance to drop his Fiery Warhorse mount. By sheer luck, it dropped for me on my first ever solo kill!
[ame="http://www.youtube.com/watch?v=2dEWkvNT7TM"]YouTube - Hunter Solos Attumen the Huntsman[/ame]
And here's how to solo the last pack of trash, for those who are wondering:
[ame="http://www.youtube.com/watch?v=d3nbzfsN1no"]YouTube - How to Solo Attumen trash[/ame]