Fixed it for you.
Again, this is not a need. It is a want. If the raid can be run without a class, then that class isn't necessary.
I've been in groups like you want with the "optimal" mix of classes and frankly, they fail more often than they succeed because they lack versatility. The options to handle something unforseen are far fewer. Last night, mobs were spawning from the portals on normal faster than I've sen them spawn on elite. Without multiple casters, it would have been much harder to handle than it was.
Multiple casters makes part 2 much easier depending on what you draw for mobs. I believe at least two is far closer to optimal. Multiple mages in part one means that one can roam and one can stay with the portal group, ensuring that your insta-kill ability is not spread too thin, though this can be done by clerics since there is little healing to be done in part 1. In part 2, their crowd control and damage spells are very helpful.
As much as I love my bard, she's not necessary to run shroud. It's nice if the casters can save mana by not having to cast dance balls, but not required. If it gets to the point where we're depending on her for healing, we're hosed. All she carries is the cure serious spell and some wands/scrolls to self-heal between fights. If all she's wanted for is songs, I'll pass on the group. That's boring.
My cleric does as much damage to the pit fiend with one cheap, unempowered searing light as any melee with a transmuting weapon. I find that one-two traditional melees is sufficient in a party stocked with decent rangers.
With decent players, no class is "necessary." UMD rogues heal just fine. I'll take Brenna, Rameses and Kiranselie (all rogues) over tanks any day and twice on Sundays. There are people who run casters and rangers in the same category. I'd rather have those players on whatever toon they want to play because they are good than I would anyone else in a specific class.
I was told repeatedly before I entered the Shroud the first time that the "optimal" group was four clerics, four evasion melees, two rangers and two casters. I think I've seen that work twice. A successful raid in the shroud or anywhere else depends on the players, not the classes.