Typically now I run two big units of Bloodletters, Skulltaker on a juggernaught, Herald on a juggernaught and a unit of six Blood Crushers. Usually I run both Heralds with the squad of Crushers for a deathstar but every once in a blue moon I'll detach the Heralds so they can join the Bloodletters. Typically this is done versus an army with huge swarms (20+ models per troop). You might see swarms in ork, Nidz or IG. It turns out the IG horde is the one that should set off red alarms. The IG horde will pack lots of guardsmen with three sergeants equipped with power swords and a commissar equipped with a power sword. The unit is Stubborn and the commissar can execute one of the sergeants if they fail a Ld10 morale check. I have watched these super mobs grind through a lot of nasty assault units. Basically they are the ultimate tarpit and those power weapon attacks really add up quickly. There are two ways to beat this unit:
Shoot the heck out of them, or
Hit them with a whole lot of attacks in close combat.
If you run KDA you have a bonus versus the IG monster mob, all your attacks ignore armor saves. The reason I run the Heralds with my Bloodletters versus these types of units is twofold:
The mounted Herald is T5 so the mob can only wound him if they roll sixes... He is fairly impervious and attacks directed at the Herald draw off from the Bloodletters.
The Herald has more attacks so you can more quickly cut down super mobs.
Versus a super mob it's all about wiping them out as quickly as possible. The longer you remain in close combat the more they are pulling you down while they club you with sticks and stones. Super mobs are cheap pointwise so it's typically a bad tradeoff versus them in close combat and they will take down most elite assault units quickly (including terminators). Most players don't recognize the threat, they charge in with their best assault unit and get taken to the cleaners. The IG super mob is the most dangerous since the unit is Stubborn - if they lose the combat they don't have to take additional armor saves due to No Retreat. Gaunts and Orks are typically fearless so they will have to take the additional armor saves and they end up dying almost twice as fast, plus gaunts do not ignore armor saves and one Nob swinging a power klaw is not nearly as brutal as four power weapons.
You can also charge a Soul Grinder into a super mob but that is basically a stalemate plus you can no longer shoot.
It's important to remember during any game that a Herald can always detach from a unit it's joined with and join another unit or solo assault. I have had plenty of games where I will detach a Herald from the Blood Crushers and join a unit of Bloodletters or launch a solo assault. Typically the more units you can assault the better.
So finally let's quickly do the mathhammer for a squad of 15 Bloodletters with and without a Herald charging a squad of 30 guardsmen with three sergeants (power weapon) and a commissar (power weapon).
15 Bloodletters charging = 45 attacks, 30 hit -> 25 kills
If you add in a Herald that is 5 more attacks for 3 more kills.
It doesn't seem like a big difference but in reality you will rarely ever be able to charge with a full squad since they are going to draw some fire on the way in. So if we were to repeat the same exercise with 8 Bloodletters that is much more realistic:
8 Bloodletters charging = 24 attacks, 16 hit -> 13 kills
If you add in the Herald then again that is 3 more kills for a total of 16 dead. You have roughly cut the mob in half. This is where the Herald can really make the difference. The mob is not going to break and the Herald is fairly safe since the mob needs to roll sixes to wound him. Basically the Herald will account for three kills each round of close combat and it adds up fast. Against orks they will most likely break the next round of close combat, against gaunts they will most likely be dead the next turn or break as well if out of synapse range. Let's do the mathhammer to see what happens with the remainder of the IG unit versus Bloodletters and a Herald (assuming the IG player is smart and stacks all their attacks versus the lesser daemons):
14 guardsmen = 14 attacks, 7 hit and 2 wound -> 1 dead Bloodletter
3 sergeants attack = 9 attacks, 5 hit and 2 wound -> 1 dead Bloodletter
commissar attacks = 3 attacks, 2 hit and 1 wounds -> 1 dead Bloodletter
I have been generous with wounds scored by the IG mob in order to be conservative. So you have 5 Bloodletters left plus the Herald versus 11 guardsmen, 3 sergeants and the commissar.
5 Bloodletters = 10 attacks, 7 hit -> 5 dead guardsmen
Herald = 4 attacks, 3 hit -> 3 more dead guardsmen
3 guardsmen = 3 attacks, 2 hit -> 0 wounds
3 sergeants = 9 attacks, 5 hit -> 2 wound, 1 dead Bloodletter
commissar = 4 attacks, 2 hit -> 1 wound, 1 dead Bloodletter
3 Bloodletters = 6 attacks, 4 hit -> 3 dead guardsmen
Herald = 4 attacks, 3 hit -> 3 dead sergeants
All that is left now is the commissar and he will probably score no wounds so the next round of close combat you'll kill him too and you are left with 3 Bloodletters and the Herald. If you play your cards right it's possible you'll be sitting on an uncontested objective or gained some kill points. In my mind that's a good trade in death. As you can see from the math hammer above the Herald makes the difference; without him the IG mob would probably win and you lost a scoring unit in the process. It all comes down to a mega battle of attrition and the faster you can grind through the uber block of puny guardsmen the better. You will also find that most IG players will concentrate their power weapon attacks on the Herald which he can probably shake off since he has 3 wounds when mounted on a juggernaught.
The IG super mob is no joke. They can take down assault terminators, Wolfguard, genestealers and plenty of other nasty units. You have to respect what they bring to the table. Remember that Soul Grinders are equipped with the equivalent of a S6 flamer so use it against them if the opportunity presents itself. A well placed template can decimate them with one shot as it does not drift and typically IG players running super mobs don't take the time to spread out their models to lessen the potential impact from a flamer. You can also fire the Harvester as well. I will run the walker right up beside a super mob of guardsmen bit typically I won't charge him in unless I want to make sure my opponent can't shot him. I feel the same about the Bloodthirster as well. Charge uber blocks with your Blood Crushers and Bloodletters, the greater daemon and walkers should be used to take out tanks and tough assault units.
That covers it all based upon my experience. Happy Hunting!