Anh... Guys, guys, guys, do you really know Killua at all? Do you really know Killua to say that? He has already defeated opponents who were so much stronger that he couldn't damage via normal means and had to resort to a weapon which buffed him in a way comparable to how a human gets buffed by using a sledgehammer in combat and that weapon is in his standard equipment. Killua is ridiculously cunning, to the point of having a fight in which he had planned out his every step to be a trap, read his opponent from the start and was just toying with him and that opponent was so much stronger that Killua couldn't damage through normal means and would have been easily KO'd in few hits. Killua is a force to be reckoned with.
Now, how would he deal with this situation? Damn, you say, Mileena is so strong, she wouldn't need to land more than a couple of blows on the right place to KO him. Yeah, I agree, and I also believe she will never, ever land a single blow on Killua. First of all, let me remind you all that Godspeed has two phases: Speed of Lightning and Whirlwind, and when someone says that Godspeed is authorized and speed is equalized, they say that Speed of Lightning is equalized to the opponent's current key's basic speed. Now allow me to tell you a secret: Whirlwind is a speed amplification on top of Godspeed's basic, Speed of Lightning speed. Not only does Whirlwind allow Killua to repeatedly blitz opponents who were not all that slower than Speed of Lightning, it gives him instinctive reactions to use that augmented speed to the best of it.
Killua has the electric warfare to keep hitstunning Mileena while never taking a single hit, to lay down metaphoric traps in every move he makes until the "stronger" woman is taking such a ridiculous, shameful and hardcore beatdown that her strength looks stupid and the power she seemed to have is painted as weak. Someone watching that fight might even think that Killua was stronger from the beginning, that's how clever his strategy designing is.