If you want to play lawt, I can tell you a few things about Undead that I've learned:
Pixies, Gowen rush, certain Wizard Kingdom mods (weak to others).
Basically anything that depends mostly on attacking to deal damage and control the field
Undead hate (Max HP stuff like Talisman of distruption, etc.)
Return (then again, who DOESN'T have a weakness to return?)
Undead guys have the ability to keep permanent transformations on their unit stats across deaths. So if you give a undead +10 to attack, it'll keep that even if the enemy manages to kill him. Their close skills mean they don't cost SP to rez, and with units like Priestess of Madness, and Lich, which can return units from the cemetary, they can continue it "almost" indefinitely. (Watch out for that "almost", it's a killer.)
However, this works both ways, because if some one gives them a -20 attack debuff, or a MAX HP debuff, it screws em over.
Undead players have to leverage this by forcing their opponents into a slugfest. Grimbane provide grim protection. Dark Confederation protects against max hp hate (important!). Skeleton archer gives range. Pure terror/Sacrifice instant kills cards.
In terms of file tiers, they aren't OPmana. But they are up there. So if you have the cards and want to try them out, they are definitely worthwhile to play.
Might as well introduce Magic Dolls too.
Hm.... I remember reading on the forums that Magic Dolls, both Command and Normal Variants are very strong, but that no one has ever mastered them.
They depend heavily on synergy, weak early game, and have only average base stats, but once they get rolling they are very strong.
Normal magic dolls play out as a rush file, usually starting out with an Undead to tank, and shades, to proc SS for SP gen and field control. They transition smoothly into mid-game with Sumer to provide rez and buffs, while Magic Doll Mobility protects against Grims, and Soul skills. They are a decent rush file, but I personally prefer WK or Pixies (less SP management required). You can substitute Magic Doll Exorcist for an undead tank, but she and undeads don't play nice with each other.
Command dolls also start out with Undead tank, but the similarities stop there. Instead of Mobility, they toss out White lily and Scarlet Rose. This file is dependent on abilities, so they correct usage of abilities and good SP management is a must. They don't have sumer, so they have to use parts recycle, and they are heavily synergistic. You need to keep them together. Violet x2 is used in both files to provide buffs on levels and atk.
Overall, Magic Dolls are difficult to play, but rewarding to master... if you can, that is.
When the end comes, and enemies are rushing in from all sides, I will be at your right hand.