I mostly agree with the Mattler, but I think the Ulthwé attribute is a bit underrated, as it is always ON, even against mortal wounds. It helps a lot when holding on to objectives and makes eldar less of a glasscannon.
Scorpions and Karandras actually have some nice mutual benefit, try tossing an Autarch into the mix and the number of (exploding)hits would increase even further. The Avatar of Khaine has some nice combos with the scorpions as well.
A large unit of reapers is common in competitive lists now, either with forewarning or with Yvraine.
The 6+ effects of shuriken weapons works great with doom, not so sure about the 4++ from Asurmen though, as most weapons firing at Avengers have no AP anyway.
"Quicken" is a great power, almost rivaling "Doom" but for a hemlock I think Jinx is the best power as it even works against invulnerable saves and due to the hemlocks speed and short range makes it a great way to "paint" targets.
As for Dark reapers or War walkers, take both, but I would rather have Brightlances than starcannons on the War walkers.