I think you're missing a few key notions at work here.

1. You have a lot of ranger experience, more than 90% of the MUD, probably. When you have that sort of experience, you become aware of the class's weaknesses and know how to exploit them. It makes total sense that you were able to take an assassin and kill a bunch of rangers. You know how they work.

2. You're giving assassins way too much credit. In skilled hands an assassin can be extremely dangerous. For someone who is new to the class through someone who is average with the class they will lose against any skilled player playing any class much of the time. Obviously lowbie assassin against lowbie invoker goes to the assassin, but by the time you're at hero any skilled player should be able to crush most assassins.

3. Mark of the Prey is an awesome skill, but you seem to be neglecting a couple of important limitations it has. You can't stalk your mark, so that entire avenue of your arsenal is out of the question. Secondly, if you're marking someone that means you're not using another command. Most of the time for assassin v. ranger that means you're in civilization. If you're marking him you're not tripping him, which may (depending on how far you are from the woods) be the wisest course of action. By marking him you're basically expecting the fight to go into the woods where the ranger (despite what you say) has the advantage. Now, if you're marking in combat you're giving up a lagging move, a damage move or a malediction to land that mark. Anyone who's played a few assassins will tell you that often times one command is the difference between winning and losing a fight, so that's big.