Well, let's do a Rare recount here against the Witch Elves+ cauldron:
Terrorgheist: dies due to poisoned attacks. Screams will do the lowest of damage, due to their general's inspiring presence of 10 or so. Better send him off elsewhere.
Mortis Engine: same as above, only that it actually has a save. Now, the witches can also use the Flaming Banner and get... hot poison (acid? haha). Still, gives our units a regen save and it's not all that bad. Screams will do poor damage here. Impact hits will actually hurt some elves, better charge them on the flank with this one.
Cairn Wraiths: not that bad since they are 3A each, Ethereal and wound them nicely (2+). Since they are immune to Terror due to frenzy, they'll be a little hard to hit, but a unit of 5 wraiths can dish out 15 attacks, not bad. The character on top of the cauldron will have a Magic Weapon though, and with so many attacks, the wraiths will die before they can hit. A Hero Wraith or Banshee champ can tie her up for a round.
Black Coach: our most armoured and thoughest model by far. It will receive very few wounds, but can only dish out 5 attacks, 3 of those should wound easily and 2 from the nightmares with 3+. The thing is, Elves still have ranks and a standard, so... the coach must do ~4 wounds per round to keep himself alive. Nice if it's como charging with another unit (Black Knights maybe), or if a Spirit Host is fighting them on the flank/rear for added CR (and safe from the Death Hag).
Varghulf: He has half(ish) the saves the Coach has, but has a much higher potential to damage them too. Has Hatred and Thunderstomp. A very high T and 5 attacks with the same WS. He is very mobile too, so he should get flank/rear charges easily. Still, against that many amount of poisoned attacks... just believe in his Regen save.
Blood Knights: this is the Unit that I wanted to get to. S3 hits will still get them full armour save. Blood Knights have higher WS, so they can hit them easier. The only drawback here is the Initiative. True, elves have ASF, but Nightshroud could thake care of that, still even so, they are I6!!! so they still strike befor the knights (but no re-rolls, which is very important). Melkot's Miasma is key here, also the elves must lose the first combat, so they lose frenzy. Dispeling their bound spell prevents them from regaining it. A vampire Lord/hero there with the nightshroud can dispatch the Death Hag there (since she's the only one who can have more Strength with a magic weapon, at the cost of losing her poisoned attacks).
So... yeah, Blood Knights! haha. Black Knights are also a good (but from the Special section), have fewer attacks/WS, yet the same armour save. Blood Knights can also take dreadspears with ease. They can also wreck Medusae and Hydras (no regen save now.. and their attacks decrease as they lose wounds) on the charge. Kharibdis is another matter. It can die... since it strikes after the Blood Knights, but has a very high WS for a monster, and a lot of S... so the vampires better kill this while they can.
Against that many enemies... the Knights are actually good against DE. What you would pay for 2 terrorgheists you can get... 8 Knights or so.
I'll try this next time, and tell you about it, haha