Against white, for me, it didn't seem to matter what units you use against them in the lower ranks, as long as you get a ranking list, powerup past that on your favorite units (but not too much), and THEN hit them. In the higher up ranks though, I find that having units that mostly have ranged attack (or fly) are nice, as well as having something on the ground that can take hits well (*cough* unicorns *cough*)or dish out damage (Sirenes) to white mages foolish enough to use ground fodder. If you have Rippers, Sirenes AND Nymphs, you can also use Monkey Brains as your item to take advantage of the fact that they all have a psychic attack.
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white is the most easiest color for me to handle... i finished 10k land and ranked 100 below last reset with 8 heroes white is very unstoppable... the only stack i want is in black.. my friend which is veteran in black recommend me the stack zom/lich/vamp/mindrip/ fodder.. but still i'm helpless.. what to you think?