tldr.... hehe, just kidding.
Anyway, thanks for the detail! I'll go over this step by step, and let you know where I see problems.
Okay the 9th level fight was like this.... The DM put this spider looking thing in front of the castle I needed to get into. My wizard friend was in the castle attempting to exit, but the door was blocked by the beast. I had my friend run up to it (despite his claims of WTF are you crazy??) but I assured him it'd be fine. Once he had the beasts attention, I stealthily jumped from the bushes to use trick strike w/ sneak attack for a +11 to hit, and a roll of nat 20 meant i crited (since my 31 would have hit, it meant max damage) and I dealt 37 damage. (2d8+4 + 3d4+5)
OK, 37 is definitely a lot of nice damage, but it's pretty reasonable for critting on a Daily. (A fighter would likely do as much if he crits on Brute Strike.)
I popped my action point to throw torturous strike next, which I rolled a 17, making 28 total and hitting its 25 or 26 AC, i forget which. I dealt 13 damage (2d4+9) because I am a nice high rolling lucksack.
OK - was combat
actually started yet? If this were a surprise round, you can't spend an action point and only get 1 action. Otherwise, yeah, dropping an encounter power right after a daily, along with an action point, will
also tend to do a lot of damage.
.... Another 10 or so damage dealt saw its way to my turn, where I decided to use Riposite Strike (successfully with my +11 to hit) for 8, and then popped my Action point to do another 7.
There's a problem here - you can only spend 1 action point per encounter.
The DM looking flustered attacked me finally for more than half my HP, only to be surprised when i told him what the effect of riposite strike was... I'd get to counter attack now. I missed the first, but the second saw its way through for 5 damage and the beast fell to the ground with a mighty sigh that nearly crushed both the wizard and myself.
Two problems here. Both involve your immediate interrupts.
(1) You technically get to attack
before the creature attacks, since it's an interrupt.
(2) You're limited to 1 immediate action per combat round. Even if you use Riposte Strike twice, you can still only Riposte once.
Our DM is new to DMing, as this is his first game to DM in 4e or in 3.5, where I have DM'd and played for years in 3.5 and played one game of 4e before this as well. Don't say something is impossible, because in a game where luck reigns just as much as skill and tactics, anything is possible.
Well, no doubt there was a lot of luck to it.

It wasn't a level 9 solo, though... in fact, I don't have any idea what it was, since most level 9 creatures have more than 72 hps. There were a few rules hiccups which probably really changed the encounter balance. Don't stress - you're still learning the system - but it goes a long way to understand why your character seemed so unbalanced.
EDIT: Also, it's good to keep in mind that the 4e combat assumptions are way different from 3e's. Facing off against solo foes is uncommon, and definitely not the default assumption... unless they're specifically-designed Solo creatures. Stuff like this is one of the reasons... A level 9 creature is 400 xp. A party of 5 characters should be facing an xp "budget" of 500 for even a weak encounter. With that said, level 9 foes are probably not good to throw against low-level parties because of their crazy defenses, but alone they're not even a minimal level 1 encounter.
-O