I'm sure I have lots of ground to cover till I am able to play an assassin with full potential. But that said, I'm not a new player and have played many builds with very different playstyles (barbs head in battle, sneaky rogues that never get aggro, squishy wizards that manipulate the field from behind, tanks that try to take aggro off allies, rangers that kite monsters in a circle for the party or lining them up for IPS, etc.) I know very well when to get and when not to get aggro as well as how to position myself on the battlefield.
But nobody's perfect. When a great number of things are happening all at once in a quest, it's tough to keep on top of things.
Considering the melee players were at center and the ranged players were kiting, getting to their position can be tough. But I did have many successful assassinates. Several Jariliths and 2 Hezrous. I was especially happy with the Jarilith ones as they can cause a lot of pain.
The problem with that particular Hezrou was that I wasn't expecting it for a while (because there were spiders still to be killed), but the melees at center were in a hurry and killed the spinner which spawned the Hezrous. At the time I thought I had the ranged on my side, but I was wrong.
What would that accomplish? I know the quest like the back of my hand. But with champions and fireballs and walls of fire and cleaves and disco balls all going on, it's hard to know where everything is, wouldn't you agree?
60 for a heroic spinner? I don't know, I thought that was the good enough DC for pale masters in epic levels...
Aaaanyway. I wanted to provide my experience which I'm sure others must be having. Insta kill is much easier in other classes. I don't expect it to be easy like bard, but this is too hard.