they will beat the crap outa their mana units, especially statues( i believe ). Mana Flare also wreaks havoc on heros, this is especially usefull against UD as well... you know.. they like to kill hero's. Plus if the UD doesn't invest in web or anti-air (1.. they are fools) you have a nice attack point they can't counter.
I would also try going mass archers if you have good micro and attack constantly at night :-D
the unofficial warcraft III forum's are pretty useful, as is above mentioned wcreplays.com (if you're hardcore, listen to the audio commentary? wow.. they are indepth in that)