Also, is it just me who gets completely frustrated when hearing "Dark elf spearmen"?
Comparing with our spearmen... these dark elves cost 1 point more, and get 1Mv, 1 WS, 2I, 1LD and hatred.
Even better: my DE opponents won't use them because they feel it's a weak unit in their book... Le sigh.
Please GW, please do DE next. Make Hydra's more expensive. Nerf Pendant of Cheese. Make me taste those sweet sweet tears.
It's true that DE spearmen are not worth making hordes with. Expect only buses with spearmen. Actually, they are often said to be best used as sacrificial fuel for the dagger.
Back on topic,
DE hordes that you might face could be Executioners, Witch Elves, Corsairs (with the +1att standard) or, suprisingly, handbow Corsairs.
They will be buffed with Cauldron of Blood, granting Execs a 5++ before melee and a second attack (already KB) in melee.
Corsairs & WE will get KB with their 3 attacks.
In addition, Execs and WE will be stubborn within 6" of COB, that's another good reason to go 10 models wide.
The spearmen will remain behind, with the Shadows Lvl4.
In order to defeat that... er...
Warmachines, of course.
A witch hunter might be able to frighten the sorceress far away.
Pistoliers hit & run.
If it is an Exec horde, you may get good results with low cost buffed Empire horde: Execs are optimized against our knights, not against our troops.
If it is a horde of Corsairs or WE, that's the contrary, they are optimized against our troops, not our knights (except if the COB is at 12").
In all cases, kill the COB first, with magic: it's a warmachine, after all. Don't try to shoot it or fight it in melee, that's a waste.
Or play terrain: hordes lose steadfast if struck into wood or river, and COB cannot pass trhough any terrain.
By the way, DE AB rumored end 2013 at earliest. Be patient.