Wraithlords are great units

Only if you take several of them though.
And yes, they can be killed by a full squad of sternguard if they drop pod close to them, but guess what? Most of the time I've dropped my sternguard like that they end up dying in the next shooting phase. Also the wraithlords are ~100-150p each, a lot cheaper than a squad with sternguard, at least if they have backup by an librarian.
Also if you fortune the avatar and run him towards the enemy, they'll have a hard time ignoring him, and even if they spend 1500p worth of shooty units just shooting at him for one turn, he'll probably end up living to see the next turn.
But yeah, wraithlords aren't unstoppable. Anti-tank weapons work wonders on them, as they are often Ap3 or better and hi-
str. As the previous poster said, you have a hard time getting cover saves, which means if it hits and wounds, you take a wound.
IMO mass volume of fire isn't effective enough versus wraithlords, due to their hi toughness and 3+ save, but anti-tank weapons will make short work of them. You can make do with poisoned weapons, but the 3+ save is going to save a lot of wounds before dying.
Oh and the eldar player should have some close combat units backing them up, because if they get hit by close combat specialists they might be in some trouble. So if he doesn't have them backed up with anything, just assault them with power fists and hammers.