Here is an overall strat for undead based on my experiences.
Undead VS NE:
Hero: Lich(frost nova), Dreadlord (vampuric act/ sleep)
Ground support: Ghouls, Banshees(posses), Abombs.
AIR support:Crypt fiend, Gargoyles.
Note on battle: NE have alot of air units, and Chimeras are one to watch out for. Anti Air is good VS NE because of its several air units like hippo/rider, druid of talon and chimera. For ground battles, Necros are ones to avoid due to dryads, so banshees are a good way to counter huntresses and dryads. take the huntresses and kill the dryads. Abombs will slice through any magic caster like druid of talon or dryad. it is a good match with the claw, but u should posses any claw or huntress.
Undead Vs Orc
Hero: Dreadlord (sleep, carrion swarm sleep infernal)
Ground Support:Necros, Banshee, Crypt Fiend, Abomb.
AIR support: CRypt fiend
Orc have crappy air, so AA is amlost not necissary. a good necro and banshee is good. Possessing taurens and shamans can be extremely fun. as for Taurens and Shamans i can only suggest Abombs and banshees with skeletons for support.
Undead VS HUMAN
Hero: Death night(Death coil, unholy armor) Dreadlord(Vampuric act) Lich (Frost nova, frost armor)
Ground support: Ghouls, Necro, Abomb, Frost wyrm.
Air: Frost Wyrm, Gargoyle.
Human suck all around, but a paladin can hurt them. this is why all three heros are good in my suggestion. Wyrms are a good unit to use gainst Gryphons and knights. Raise dead and a good use of ghouls can be devistating for fotman and rifleman. Abmobs are a good against Kngihts.
Undead VS Undead
Hero Dreadlord (sleep, carrion swarm, Vampuric act Infernal)
Ground support: Ghouls, Necors, Banshee.
Air support: Crypt Fiend.
Undead, i dont suggest building abombs, because possession can be deadly. i propose less expensive units, so u can poses his abombs. as for hero, i only suggest the Dreadlors.
WEll this is sad, i spent forever on this and i could add much more. u can use this guide to use against, or for playing undead. have fun, and post suggestions, and critism.