Lol, whats up ugly my man
Anyway, to the topic at hand. The cold hard truth is that your rage spec is just a straight up lousy counter to assassins. From reading your post, I am having a difficult time trying to figure out what spec this assassin is. You say he puts dots on you, which leads me to say madness, but you also describe a powerful opener, and some really big burst, which comes from deception. Madness does have some burst potential, but not at the levels your talking about. And deception has no dots, unless they take 2 secs to cast a crushing darkness, which I'm sure you would have noticed. So my best guess is that he was playing a 14/27 hybrid or some variant. But either way, it just sucks to be a rage warrior vs assassins. Madness can dot you up and just kite all day. Deception knocks you on your butt and unloads. You mentioned him resisting your first smash, which can be attributed to force shroud.
I mostly play deception, and am more familiar with it, so from here on in ill speak about that spec. First off, we get a straight 30% AOE damage reduction, plus another 25% from Dark Embrace, which will likely be up, especially in duels. So your OMGWTF 6k smash was just turned into 2-3k. And that pretty much kills rage spec right then and there, without that big hit, your done. And then deception will follow it up with multiple 5-6k hits of their own. Deception assassins are among the hardest bursting classes in the game, if not number 1. And yes, they do have a lot off cc, with Deception getting an extra 4 sec mezz in addition to everything else the class gets. Honestly, the matchup is just so far in Deception assassins favor, that I really have very little help to offer you, other than do not be caught alone, as that is where Deception thrives, in 1v1s. Pop your cds, use freezing force and crush to slow them as much as you can, try to kite if you can. Popping those cds will help with the initial burst.
At the end of the day though, Rage spec is the new FOTM, because OP or not, its easy, fun, and gets big results. And Deception assassins serve as the best counter to them right now, with their ability to resist or shrink that 6k smash, and then retaliate with multiple huge hits of their own. Rock, paper, scissors kind of matchup here.