Undead? Easy.
Get breakers.
Unless things have changed 99% of the time they go fiends and DK. Breakers totally screw that up, ESPECIALLY if they try to get destros.
You get foots in the beginning. Tech, then drop 2 sanctums. Then pump the breakers.
Breakers, will do double damage against fiends yet take reduced from fiends. Breakers also move fast. Their feedback works AMAZING on the mana of heroes and they can NOT be nuked. Finally, if you see a statue, breakers will kill that statue FAST with feedback.
Key units for HU:
HUvORC - gryphons: pwn burrows and melee units
HUvELF - riflemen: pwn hunts, dryads and casters
HUv UND - breakers: pwn fiends, stats, unnukable (if that's a word)
If you see UD going fiend, DK and stats, you are at a win-win situation if you mass breakers.