First, realize that said 50 Bloodletters + Herald are costing your opponent roughly 800 points assuming Full Command and obligatory Banner of Additional D6 Charge, plus some gear on Mr. Herald.
With that in mind, 10 Crypt Horrors, 5x2, cost half that, and will put a serious hurt on them, but NOT stand up to them and win the combat alone if the opponent is at full strength.
The BLs in Horde will do on average 25-26 hits with the Hatred, half of those which will wound, and IF the Herald either doesn't have Firestorm Blade, OR doesn't do a Wound ahead of the Horde's Init, you save 1/3 or 1/2 with ME. This means you take anywhere between 6 - 13 wounds, depending on the above variables.
Assuming the worst, you lose 4 Horrors, meaning 6 strike back with 18 attacks, doing 9 hits (3 poisons), 4 of the 6 wound, and 1/3 of the 4 are Warded away, leaving you with ~5 wounds inflicted. 5 Stomps probably add another 1-2 wounds, meaning in a worst-case scenario, your opponent does 12-13 wounds, you do about 7, and lose combat by another 6-8 depending on his static CR.
As you can see, NOT a good result. Now, assuming no Firestorm or no Flaming wounds, you suffer 4 fewer wounds, and also gain an extra 3 attacks from the not-dead CH, which say if you're lucky adds 1 more wound. This means you go from losing 12 - 7 on wounds, to 8 - 8, and only lose by Combat Res. A much better result, and sets you up wonderfully for a counter-charge, Invoking, or Buffing in Magic.
Here's the BIG key - do NOT charge them. Let them charge you, which they will / have to, to earn points. Just march up your CHs in front of them, and let him charge. This gives you only 1 combat on his turn, before you get a Magic Phase to buff up and heal the Horrors. You could charge, and hope to get a buff on your turn, but then you need to survive 2 full combats before you get a chance of healing the Horrors and risk losing the entire unit this way.
Again, the Horrors clock in at less than half the 50 BLs + Herald, so don't be disheartened if they don't smoke them. Their job is to survive the initial punch, throw an equal punch back, and grind them down in subsequent turns once the Hatred wears off. Hopefully you can get a supporting unit like Ghouls into the Flank on your turn, but if not, try your best to Invoke / Vanhels the Horrors, or Soulblight the Letters. Also, even if the Horrors die, a 40-man Ghoul unit should be able to take down the surviving Bloodletters, who should be somewhere under half-strength after the CH battle.
Obviously, a Mortis Engine also adds to your odds in this situation, as your Regen gets better, and you also cause some minimal damage to him via Reliquary.
At the end of the day, Demons are tough BECAUSE Bloodletters are so underpriced, but hopefully the new book changes this soon. It's also part of the reason Crypt Horrors, and double Crypt Horrors, were so prevalent at ETC!
- Larry
