Here are my thoughts on it. ather than make some big flashy assassin/merc, I think they should create a highly secrative and pretty uber new aspect warrior, a group designed to go along with an eldar army and take out key targets. Give them great stealth and good CC skills, basically they slip in, take out something like the HQ or heavy support, then vanish back into the shadows only to strike again.
They shouldn't be an eldar sniper, that's what the rangers are supposed to be for. They should also be able to crack both vehicles and infantry since they target ammo dumps and the likes as well as people.
Finally, regarding how powerful they are, I think they should be more powerful than imperial assassins. Look at it this way, an imperial assassin is trained in their role for about 20 years, training constantly day in day out. These a re a group of eldar who do the same thing, expect they do it for hundreds of years instead of a few decades. Which is why I think harlies should at least get an assassin dodge, but that's another gripe al together.