"How should you play as Human against an aggressive UD who goes DK fiends (and possibly has map hack)? I lost a lot to someone like this on GGC where it seemed he always had map control. What's the best way to kill fiends when the UD is good at microing them? Or what should I do?"
First things first , always assume that your opponent is good at microing, cuz really cant judge, every battle is different!
Secondly it depends on what opening you use , assuming you are in this page you either use bloodmage + footies aggression opening or bloodmage fast or bloodmage slow tech.
With this opening you easily defeat single DK and drain his mana pretty well too, no matter what the situation is dont engage undeads army with only footies ( unless he is creeping an expo or so ) . Pretty much just run arround while you militia creep a little bit and get mass rifles from 2 baracks, point of rifles is to help you creep your level 3 on bloodmage so that you could go harassing in tier 2 ( flamestrike on acos or ziggurats ) while your paladin creeps fairly easily with huge rifle force ( mass rifles gives you an early tier 2 edge ) . Once thats done you simply camp at your well built base ( make it open and have few towers in it too ) while either getting tier 3 or making yourself a tier 2 expo , of course harassing with your heroes and maybe with rifles until those things done ( dont overdo aggression if you expo tho' ) . Pretty simple really, no matter how good he microes you can micro good too and its somewhat easier for you , you have siphon mana and paladins holy light and divine shield, can negate novas and lose nothing. Just be aware that you dont need to rush into things, with BLM paladin you are more of defencive than offencive!
Bloodmage or Paladin fast tech (17 or 18 tech)
Its pretty similar to above , you just dont militia creep while harassing and rather focus on getting huge ammount of rifles in a flash so that you could creep biggest camps before undead becomes strong enough to negate you doing that. Later you can expo or tech tier 3 or even do both if you took fair enough advantage with your early tier 2 power. Rifles/gryphon mix is capable of tackling UD head on if you have sufficient micro, but micro will be alot easier than it will be for undead , mostly because you have imba paladin and flamestrike.
Bloodmage slow tech
This build isnt really that useful vs undead in solo, but if you do it , you should go for extremely slow variation of it ( 23-24 food tech ) with blacksmith and baracks before teching, I think I posted guides about this build , but if I didnt just post a comment and I'll dig it up. Anyhow, with this build you can get alot of rifles with upgrades as fast as 18tech , but be able to militia creep massively in exchange of slower tech ( I think its 1:30minute difference lol ), its pretty much self explainatory, you can get lvl 3 bloodmage in a flash, have a safe base and have massive miltia and rifle force in tier 2 early, meaning you can creep lvl 3 paladin super quickly too, its very risky tho'. What I would do is, I would harass w/ bloodmage while rifles and militia creep to level 3 and then maybe go into zeppelin drops in tier 2 while my paladin is in production and later just do the same routine as before. Tech or expo and always harass, never engage (atleast not until you got dream army), just defend.
Btw , fiends die if DK has no mana, can always have bloodmage in front with siphon mana and let him be focused , cuz you have paladin who can heal him easily. And fiends die pretty well to rifles really and flamestrike, and ofc theres holylight too, could improve your own micro too, micro is fun
By the way, I am able to get ethernet cable at nights, so Ive been playing some 2v2's with contempt , lag free at last, so there should be more reps coming up I guess. Cant really play any tournaments tho' , cuz there are none at nights obviously