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!
Trash Solo:






