Do the clerics have time to prepare?
If they do, have them cast Prayer/Bless/Resistance/Protection spells on each other before the beginning of the combat. Possibly even Aura/Shield spells to get even more buffs. Holy Defense from Channeling Companion is a great list in this sense, a cleric with access to it can easily get a +35 or more bonus to his DB.
Have the less powerful clerics concentrate on defending the more powerful ones. Use things like Airwall or Waterwall (Barrier Law) to slow down melee fighters and shut off ranged ones.
Since they're cleric of the dead, there should be plenty of corpses around. Sure, having a couple of undead bodyguard ready is good, but don't understimate the Animate Dead spells. Use them to keep melee fighters away from the main casters. The animated dead won't probably last more than one round against a good fighter, but can be immediatly re-animated (unless hacked to pieces, but that takes time...) if they're defeated. Higher level clerics can even animate more than one corpse in a single round, making them particularly effective at distracting the fighters.
You want at least a cleric to keep an eye on the party casters, using Cancel/Dispel spells (from Spell Defense) to distrupt their spells.
Dark Channels is your main offensive list, but it's so good you won't need anything else. Seriously, it's possibly one of the most powerful spell lists in Spell Law.
Dark Sleep is usually all you need. You can use it as a cheap version of Absolution (put target to sleep, have one of your animated corpses finish him while he's helpless) or to capture the PCs alive.
Black Channels and Absolutions are great for their high penalties to RRs, but just an overkill most of the times, keep them for really though foes.