It definitely sounds like it to me. What is your build? Because my experience with paragon+ Oassassins is they are practically useless. While I have found that Rogues simply become amazing and Rangers are practically on auto. Unless both of those characters have made absolutely terrible decisions, I can't see how the Oassassin could outdamage them. Unless you're heavily charopped and they are not - which I wouldn't be surprised if that was the case.
In the case of the ranger, he's horribly designed.
The rogue (pre changing to essentials) was ok. He had backstabber, and daggermaster, and knockout. He was able to do some good damage on occaision, but the PC is played a bit ... risk averse. So he doesn't get C/A every round. [He is now a thief who has that problem solved.]
As for my Ossassin, it's a changeling with 22 Dex, 20 Cha and is using the bonus damage vs. isolated target build. It's got a shadowblade, iron armbands of power and bloodied guantlets. More importantly, it's multiclassed into rogue, and has both "extra shroud" feats (the generic "get an extra shroud this round" feat, and the "get a shroud when you changeling trick"). So, on the first or second round of each fight, I ussually drop a 3d8+7d6+28 on a target, with the attack being against Reflex and, thanks to c/a, being about +21 to hit. Once per day, with the marked for death daily (and an action point) I'm able to put all four shrouds on a target two turns in a row (my feat enabling 2 ways to do 2 per turn, and each shroud counting double). Often if I'm "out" of easy ways to drop extra shrouds I'll build up shrouds while turning into a pseudo defender with the garotte daily. It does help as well that (a) our party has 3 exceptionally stealthy people and (b) I have the daily utilty to make the entire party stealthy and (c) I took the admitedly 'weak' feat that makes my shrouds unnoticed to people who can't see me. So on a few occaisions, I've been able to build up shrouds before the encounter started, and once or twice was even able to shroud up and coup de gras a sleeping target.
Admitedly, I've seen some powerful strikers, but in terms of damage available every encounter (crit or not), he does have a solid spike. Then again, I would gladly switch him over to the Executioner variant, as the 1/encounter spike is simplified greatly, and the garotte power is made more available. As far as shade form goes, I don't really use it that often, but then, we don't end up with radiant powers thrown at us too often either.
The Ossassin, had it been a vampire, would have been toast at the start of the game though, as we had a skill challenge where I failed at every edurance check costing me surges. It wasn't that my endurance were bad, it was my dice.